Did yo set the device to Factory Default an edit the Settings maual without a restore? In my 5 Year experiece with lowbudget TP Link Devices the restore over diffrent Firmware versions was always an...

@MrSlash Hi i had to change the SIM Card. I fgured out,that this older SIM Card might be the Problem. With the new one the Connection is stable. You might use your Card in a diffrent LTE Router to...

@MrSlash Same Topic with my MR 600 V1 EU Diffrent Providers, (A1 and Vodafone) diffrent Sim Cards and always the Issue