This is because this board is a development board that doesn't have a built-in EEPROM. Without the EEPROM, there is no configuration data for the board and it is limited to its boot power of 6dBm.
There is a rather extensive thread on this issue on the Banana Pi forum. To summarize, I was able to patch the mt76 driver to allow it to load the EEPROM from a file, and then use the EEPROM settings from another mt7615 card which allows for power up to 24dBm.
If you want, the patch is available here. When building OpenWrt, if you are in the openwrt folder, you can get the patch and store it to the right place by:
# mt76 eeprom.c patch to allow eeprom loading from files
mkdir package/kernel/mt76/patches
wget https://va1der.ca/~public/openwrt/patches/mt76/999-mt76_eeprom_file_220302.diff -O package/kernel/mt76/patches/999-mt76_eeprom_file_220302.diff
EDIT: Incidentally, what revision of board is your R64?