Just switched to Celcom Fibre within the last month. Celcom gave me a new WIFI 6 router as well, and we continue using TM's existing BTU (https://www.unifi-tm.com.my/free-unifi-equipments/), which functions like modem. I maintain the same speed for the new subscription with Celcom, which is 100mbps.

The internet was down for less than 1 hour during the installation, and after that, things are just like before (when we were with Unifi Fibre). As explained by Celcom's agent, we are still riding on the same infrastructure. No problem with speed or stability at all.