Strange Wireless issue with Two identical routers

I have two identical Xiaomi 4A-Gigabit v1 routers.
I have flashed both of the with the customised OpenWRT firmware using
the online OpenWRT firmware selector.

The first router is router A and both of its 5GHz and 2.4GHz radios
are able to transmit with the maximum TX power of 20dm (LuCi default driver value is set to 20db-100mw for the channel 40)

While for the router B the 5GHz radio TX Power is limited to 3dm (Confirmed with LuCI and real third party WiFi signal strength analyser).

For router B the option 100mw does not exist even changing the option txpower '20' in the /etc/config/wireless file for radio1 did not affect any thing at all and still the power remained at 3dm (I have rebooted the router too)

even with my smartphone WiFi analyzer I get very weak signals when I am close to the router and it drops very fast to the values less than -80db after moving 4 meters away and as a result I lose connection.

That said I am wondering why one of the routers do not have such issue and the other one have while I have purchased them from same shop and stock.

By the way I have flashed the stock firmware on the router B and checked for the 5GHz radio signal strength it was -30 db when my mobile was about 20 cm far from the router and the power of the 5GHz radio on router B was the same as router A and there were wtrong, one should conclude that there is nothing wrong with the router B hardware.

I want to know what is wrong with the Router B and why OpenWRT does not utilise the maximum power for the 5GHz channel.

I have read few posts about this issue and the posts have mentioned that there is a bug wrong with the MT7621 wireless drivers BUT if that was the case it should affect both of the routers, indeed that bug was a couple of years old too.

By the way I have to mention that I have flashed the router A with the base image version is LuCI openwrt-23.05 branch (git-23.233.52805-dae2684)
/OpenWrt 23.05.0-rc3 r23389-5deed175a5 => Working 5GHz firmware
The firmware for Router B is later than Router A (I have tried different firmwares but not snapshots) for Router B and none of them solved the issue with the router B.
Infact I have lost the Router A firmware that I flashed it in order to make a comparison.

None of the firmwares made the 5GHz radio of the router B to work properly (20dbm).

I have following questions,

(-1-) How can I solve this issue with Router B and tackle the very weak 5GHz radio on it which does not exceed 3dbm?

(-2-) How can I regenerate the firmware that I once generated for Router A with that specific aforementioned branch and commit tag on the OpenWRT website, If I spend time an regenerate that specific firmware is that going to solve the issue?

(-3-) Is there anyway to mirror the flash contents of the Router A on Router B without opening any hardware?Is the miroring the flash contents going to solve the issue?

For your information there is no country set for both of the routers A and B and I have tried setting different countries on the router B in addition to changing the channels none of them have changed the TX power on Router B?
Is there anything hardwritten on the Router B board which prevents higher power for TX?
If the factory has limited intentionaly the TX power in a unchangable way so why the stock firmware on Router B works fine with the 5GHz radio and the TX has 30dm output?

In the end I would like to know if there is any patch or software based method that I use to increase the TX power for both of the routers from 20db(100mW) to 150mw or 30db?