A problem with the LTE modem on TL-MR6400 v7 router

Installing and Using OpenWrt Network and Wireless Configuration
6

Update the modem firmware if you can, disable MBN autoselection as shown here Registration denied on Lebara Sim - #7 by AndrewZ
switch the modem to MBIM and follow https://openwrt.org/docs/guide-user/network/wan/wwan/ltedongle