My WiFi is terrible!

It works some of the time and I get high speeds but almost every single night it slows down drastically... Like it takes forever just to open a webpage... Sometimes I turn off WiFi and just use my data, that's how bad it gets! Sometimes it helps to reboot it but often it don't.

I have tried doing a factory reset with no luck, I have tried to reload the newest firmware manually, still no luck. I'm not using smart WiFi, I got it split in 2.4g and 5g. I've tried playing with the different channels and width but again no luck!

The only the firmware that worked proper for me was the original, but if I try to downgrade, it just says that it's the wrong file type... Is it possible to do a downgrade??