ZTE MF286D can't get connection over LTE

This is not what this topic is about.
You need to be on TELIAMF modem firmware already in order to use incremental updates for that base version. You will need to use a ZTE framework to flash it, but that has nothing to do with OpenWrt.
I suggest you to seek help with update at https://eko.one.pl/forum/viewtopic.php?id=21847

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