I solved the problem. It lay in one of the cables in my setup (the one from the modem to the main deco). I switched it out, and everything works perfectly again. Keeping the thread up for...

I have an issue with my Deco M5. I'm only getting 95 mbps on a wired connection (even when connected directly to the main Deco), while I should be getting 5-600. Previously, the problem was resolved...