I have Community Fibre and a Deco M5 mesh set up with the primary deco plugged straight into the Cf modem. Their engineers set it up for me when they did the install and it's worked fine ever since.
Hmmm just tried plugging the Deco straight into the CF modem (after trying to convince myself I haven't got time to be fucking around with it) and the speedtest dropped down to about 200ish on my phone. More investigation needed I think.
I have Community Fibre and a Deco M5 mesh set up with the primary deco plugged straight into the Cf modem. Their engineers set it up for me when they did the install and it's worked fine ever since.