For 90% of users, the OTA method is best. ZTE pushes updates via the carrier’s network.
After updating, do a factory reset (save your config first) to clear out any old cached quirks. That made a noticeable difference for me. Zte Mu5001 Firmware Update
Ironically, to update your 5G router, you need internet. Use the device’s current connection or tether via USB to a PC with a wired connection to download the file. For 90% of users, the OTA method is best