I don't know what your problem was but I have this exact setup Unielec U7621- Mediatek MT7621 & MT7612 + MT7602 wifi
Same router, same wireless card from same vendor. It should work fine.
The vendor responded today about the eeprom issue:
we write the right eerpom info the efuse of modules. But some user use default eeprom info on Flash.
According to this issue report https://github.com/openwrt/mt76/issues/128
the eeprom on the efuse needs to be used