@samuelgwx's idea is good. Please try running iperf3 on another device in your network over the wire as a server, and a client on your wireless device, in both directions. This can properly decouple Wi-Fi performance from your LTE performance. I'll try to unpack the firmware and check if there is anything different in that board definition file.
Edit: it is. It seems, that MF286A and R worked just fine with what's present upstream, but it differs too. board-2.bin for MF286C differs substantially.
@victor186 here is the file: https://drive.google.com/file/d/1Ze6NH8-bLN5ZDNejge2xbxrrUjqvyCC3/view?usp=sharing
Please copy it over /lib/firmware/ath10k/QCA9888/hw2.0/board-2.bin and reboot. Do not use that on any other model, it is specific to MF286C only.