My C2300 is being a massive pain, WiFi dropping all the time and when things are connected to WiFi it's unusably slow. 
 

It is linked to my sky router as I have disabled the WiFi on the sky router because it's poor in areas of my house.

All was fine, occasionally dropping WiFi, but since Friday it's been terrible. 
 

I've reset it, and even upgraded the firmware to the version mentioned above. But this version seems really old as it's 2017. Is there a newer version? I can't see one on the TP Link website.

Even if I upgrade the firmware, I'm not confident this will fix the issue, so any ideas why it would suddenly start being an issue and what I can't try to fix it please.