I have the exact same issue with a fresh purchased TP-Link MR550 LTE router bought on Amazon like this member.

I use a "Multisim" SIM card, and if the MR550 is connected to the LTE network, it rejects incoming calls which also makes my phone with the "main" SIM card do not ring anymore. I just see a "missed call" notification.

I see there seems to be a beta firmware for some models.

Would it be possible to share a new firmware for the MR550 users?

Beside that, as a way forward and feature idea, the devices should not just reject calls.

For better understanding what "Multisim" is for:
It allows you to have a kind of copy from your main SIM card and using the same contract/data plan. So you do not need to purchase another data/LTE subscription and use with up to 5 SIM cards the same contract. All SIM cards do ring if a call comes in, and the device that picks up will have that call. So if the TP-Link router rejects by (firmware) design a call because it has no call functionality, it also rejects the call for all devices, which makes the Multisim unusable. Some devices from other vendors let you configure how to react if a call comes in. This would be of course the best solution. But just to ignore the call will also work. Multisim is a featue from all main phone providers and very common in Europe.