Country Code affects transmit power on 19.07 Netgear WNDR3700

Hello, I'm new here, but can't search similar issue.
I have Netgear WNDR3700 v1 (Atheros AR7161 SoC, Atheros AR9220 and AR9223 WLAN) and its wireless signal power had reduced after upgrading from OpenWrt 18.06 to 19.07 - for example even one or two walls is enough to losing connection on laptop or phone.
After resetting settings and comparing I find out that setting of Counrty Code is causing this, when Country Code is "driver default" then signal power is ok and Wireless 2.4 GHz Device Configuration page displays "Current power: 26 dBm". If I set Country Code to RU or DE (didn't try others) then signal power is low with "Current power: 17 dBm".
All other settings are default (Maximum transmit power, Channel, Width, Forcing 40MHz) and have no effect.
On OpenWrt 18.06 I had "RU" Country Code and no problem.
Is it known issue or do I understand smth wrong or should I report bug?

This is normal and expected behavior. The country/region selection functionality is there to ensure compatibility and compliance with local laws and regulations with respect to radio frequency emissions. Please set the country code as appropriate for your location. Setting other codes may cause your router to transmit at power levels that could violate said local laws and may even interfere with critical radio infrastructure in your area.

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Germany allows 100 mW for 2.4 GHz band, I think. That should be 20 dBm.

Yes, I see that with RU code I have 2 channels more (12 and 13) and it should be set for local laws compatibility, but my experience, with other routers with set country code and with this router on OpenWrt 18.06 (and earlier), tells that on OpenWrt 19.07 with country code power level is unusually low.
At least my wireless network is unusually weaker than networks of my several neighbors %)

Check the current regdb values.

The best way to test the power level settings on OpenWrt would be to measure the actual rf signal strength when running on the stock firmware for the router (as sold in the region), then flash and test openwrt under the same conditions (Locations, distance, etc).

Basing the signal strength vs that of your neighbors is not a good metric unless they have identical routers with stock firmware. The router electrical design, antenna design, placement, and environmental factors will affect the signal strength. And, the signal meters (in%) are not always calibrated the same way and/or meaningful. Looking at the true signal strength rssi in dBm will give more insight than a percentage metric.

Well, of course I don't have probably enough knowledge and devices for true measuring approach, so issue based just on everyday use - before 19.07 I never had problem to connect router's wireless in other room... Maybe it's something else in firmware or in my specific router that didn't match well with country code, let's see if will be other responses from anybody with such hardware.