Just tried to configure a brand new hAP AC3 LTE to function as a LTE / 4G router.
But the hAP AC3 keeps saying "SIM not inserted".

The SIM works perfectly in an iPhone and has worked in another hAP AC3 LTE before.

The device is running 6.48.1 (both packages and routerboard firmware)
Also tried 6.49beta27, same results.

Any thoughts?

Well try to remove it and insert it again a few times. Also check that it's not upside down. The error means what it says.

Please connect to that unit any internet to ether1 and when unit have internet then try upgrade lte module firmware:
https://wiki.mikrotik.com/wiki/Manual:I ... re_upgrade

This helps some people.
Others use a isopropyl alcohol at contacts of sim and this fix connection.
Some others person tape office transparent tape at back of sim and then it's start be working fine (special some revision of lhgr).

Hi guys thanks for the advices!

It turned out to be a physical issue with the SIM-card connector pins, not all pins actually had contact with the inserted SIM-card.
We've bended the problematic pins and it instantly worked!

Thanks again!