Hi biangbiangmian, I got same issue. My board is U7621-1(MT7612 + MT7602 + 4G model). Have you solved this?
Hello, sorry for delay, i am unable to test it temporary as 2.4Ghz also have incomplete support (power amplifier not working and signal is too weak). I tested last week, still it is pretty unusable even in 2.4ghz 
I will try to test 5Ghz soon.
Hello, I installed the compatible firmware for the zbt-wg108 router "openwrt_ZBT-WE1326_V7" and the 2.4Ghz and 5Ghz wifi modules work correctly. This firmware runs on Linux kernel 3.10.
https://drive.google.com/drive/folders/1ZFfQQ-7KYNgKc53z_G3_8BOW8Lcl0oEp
Extract the drivers (.ko) of the wifi modules to test in the openwrt 4.14 kernel "openwrt-19.07.2-ramips-mt7621-zbt-we1326-squashfs-sysupgrade.bin" and the problem of tx power remains.
I am attaching the firmware files for possible help to solve this problem.
regards