Thanks for helping! I will try to answer all your questions.
There is no official build for this device, I installed the one that can be found at https://forum.openwrt.org/t/tracking-down-regression-led-cf-e538ac/79717/8
I do not know how to check if it has NAT offload or where can I find the settings to enable it.
I am referring to speed results from speedtest.net using WiFi, but I also have the problem when connected via Ethernet cable.
Yes, the stock firmware could reach 600Mbps when connected to the RJ45 port, and the device is advertised as a "1200Mbps AP". It is a Comfast CF-E538AC.