I had the same issues as mentioned in this thread, and I think I found a workaround.

My Deco setup is:

- 1x M4R, firmware 1.5.0 Build 20210511 Rel. 34580, connected to ONT from my network provider, using PPPoE, with internet VLAN enabled

- 2x M3W, firmware 1.0.6 Build 20201013 Rel. 22614

It used to work just fine before the firmware upgrade, and I was able to reach ~220Mbps DL / 50Mbps UL when running speedtest on my mobile phone in the same room where M4R is located, so that's pretty close to what my provider claims to deliver (300/50). After the update speedtests starts download at around 200Mbps, but then within a second or two it drops down to 1-10 Mbps, or disconnects. Or download reaches speeds close to max, but upload is throttled almost to 0. When I connect to the internet using router my provider gives, it works just fine, so that's for sure problem with Deco - and M4R in particular, because when I power off all M3W devices, the problem still exists. And obviusly I can't switch to a different main deco unit, as suggested in this thread, since I have only one M4R :)

What works for me is to enable QoS, set the limits to 300/50, and give high priority to the mobile phone - now speedtest works fine again and websites load properly. As soon as I disable QoS (which I never used before the update anyway), the problem is back again.

I hope that helps someone. I'd be happy to provide any logs that can help resolve this, I just need some tips how to do that :)

Best regards,

Kamil