Device Asus RT-AC51U
Previously use 18.06.x and 19.07.3 - work fine.
Now Firmware 19.07.4 official build download
ISP - dhcp client ethernet 100/100mbit (from Fiber to eth convertor, GPON)
(partially used google translate, sorry 
)
In the first couple of hours of use, I noticed that something was wrong.
Sometimes the internet freezes for a minute.
I opened the logs and saw something abnormal in them. This has never happened in ~2 years.
The first thing I tried was to reflash 19.07.4 again, without saving the settings.
This did not bring any positive results. The errors are exactly the same after 10-30 minutes.
Now I am rollback to 19.07.3 and all work fine 2+ hours. As usual.