Obelan Enforcer

Medium construct, lawful neutral

Armor Class 15 (natural armor)
Hit Points 48(10d6+4)
Speed 30ft.

STR 18 (+4) DEX 10 (+0) CON 16 (+3) INT 8 (-1) WIS 12 (+1) CHA 6 (-2)

Damage Immunities Slashing, Bludgeoning and Piercing from Nonmagical Attacks, Poison, Psychic
Condition Immunities Charmed, Frightened, Sleep
Senses passive Perception 13
Languages Understands Common But Can't Speak

Challenge

Anti-magic Shield. The Obelan Enforcer has a field of anti-magic that extends around it in a 10 ft. radius. This field works like the Anti-magic field spell.

Unrelenting. If the Enforcer makes an opportunity attack, it can use its multiattack action. Additionally, the Obelan can move up to its speed as part of the same reaction.

Actions

Multiattack. The Obelan Enforcer makes two slam attacks.

Slam. Melee Weapon Attack: +5 to hit, reach 5ft., one target. Hit 5 (2d6 + 5)

Anti-magic Surge. The Obelan Enforcer sends out its Anti-magic Shield as a wave of force. Each creature within 30 ft. of the Obelan must make a Constituion or Dexterity (DC 16) saving throw or take 4d8 Force damage on a failure, and half on a success. On a failure, the creature's magic and magic equipment is suppressed as if under the effects of the Anti-magic field spell. This suppression lasts for 1d4+1 rounds and the Obelan loses its Anti-magic Shield until the suppression effect ends.

The idea was to create a construct that was made to hunt mages and sometimes even employed against Beholders and similar creatures of powerful magic. It's also nearly invulnerable, and the players have to find tricky or at least more non-standard ways to fight it, but attack-wise it doesn't have a lot of options and the most damaging attack it can do negates it's anti-magic temporarily

I am also thinking of creating a whole line of constructs based on the 'Obelan' type that serve either as construct guards or strike forces for a city's armed forces or even for powerful mages, with different types fulfilling different roles

People have suggested it's around CR 3-4, but I think it's around 5 or 6

The guidelines in the DMG suggest ways to calculate CR. If you ignore the anti-magic and damage immunities then this would probably be a CR 2 or 3.

However, it does have a permanent anti-magic field and many immunities, including an immunity to non-magical weapons. As rpgstar's answer points out, these two things combined makes it invulnerable to everything except non-magical fire and acid.

When thinking about CR and what monsters to use in encounters, I find it best to think about the capabilities of the party and the wider context. For example, a monster with (just) immunity to non-magical weapons might be easy for party equipped with magical weapons and containing spellcasters, but ridiculously difficult for a party that has no spellcasters and no magical weapons.

In the case of your Obelan enforcer, it's capabilities are going to make the CR range from "moderate" to "run away and find an adventure somewhere else"!

Personally, as a DM, I would only consider using such a monster against the party if it was the main conclusion of a quest that actually involved investigating and hunting out ways to counteract it, which the party would then have to use.

For example, a chapter where the PCs infiltrate an old factory to find the enforcer's blueprints and discover that it has a switch on its back that deactivates the anti-magic field for 3-4 rounds. Or a sequence of command-words and rituals that deactivate it but only if the party can keep it busy and distracted while a PC spellcaster concentrates on performing the ritual. (And so on).

In this case, and only in this case, CR 5 or 6.

Determining CR isn't going to work for this monster

While it's possible to apply various schemes to get an estimate for the power of this monster, its set of abilities is so unconventional that its actual difficulty will swing wildly between parties and attempts facing it. I would not run this monster without significant changes, which you should fix before considering its CR.

Class imbalance

The creature's abilities make it immune to the forms of damage PCs usually are capable of inflicting. However, since you excluded Monks (in comments to the question), and Monks deal magical damage from Level 6 onwards, you have created a monster that is fiendishly hard for any party to hurt but is actually pretty easy to deal with for anyone who is lucky enough to have chosen a Monk among their characters.

Frustration

Your monster seems to be designed with the sole purpose of disarming your players from all the features that make their characters awesome. Wizards and other casters can't use their magic, the rest of the characters can't attack it for damage unless they're monks. It's bound to be frustrating. What do you expect your players to do during the combat when you have taken away their normal routines?

If you want this to be a puzzle encounter, my recommendation is that you don't, or at very least communicate very clearly that the players' usual moves are not going to work. Be generous with the amount of information you give about the monster to combat frustration.

Recommendations

The only way I can see the concept of this monster working as-is is as a very special puzzle boss encounter, as an explicit challenge for very experienced who love to test their system mastery, or after a quest that gives your players additional means to counteract its otherwise insurmountable resistances.

You could demote this monster to a more normal tier of power by simply removing the nonmagical attack immunity, so your martial characters have a chance to take it down even if your casters are disabled. It'll still be frustrating to your casters, but you can counteract that by having magic-susceptible enemies in the same encounter.

This monster is basically indestructible.

There is no reason for it to use its anti-magic surge... ever. it is (nearly) indestructible if it doesn't use it. And it would only be able to be damaged by falling damage or torches/oil, with 48 health it would destroy most parties very quickly. Unless the DM gets bored and decides that the construct feels like using its anti-magic surge the players will almost never defeat this monster. Making it only resistant to non-magical bludgeoning piercing and slashing and it could be given a challenge rating. Right now it could go up against a tarrasque (CR 30) and destroy it easily albeit slowly.