Hello,
Regarding this issue, you are having kindly test upgrading the firmware to the latest one RUT2_R_00.07.01.4_WEBUI, and recheck if it will work. It is suggested to untick the keep settings while doing the firmware upgrade. Also, kindly share me the frequency bands that your ISP operator uses because the model you are having is supporting the following bands for 4G: B1, B3, B7, B8, B20, B28A.
If the issue still persists let me know.
Regards,
Jerome