APN issues on ZyXEL LTE5398-M904

[AndrewZ]

Disable MBN as shown here: Registration denied on Lebara Sim - #7 by AndrewZ

Check/set the APN on the modem using a terminal as shown in the wiki:
https://openwrt.org/docs/guide-user/network/wan/wwan/ltedongle#lte_or_5g_modem_preparation