It still dropped the connection with the BT hub2 connected
The ONT would seize up and slowly reboot itself if left over 15 or so minutes with both the BT Hub and my TPLink router.
The difference being with the TP Link router everything would reconnect, with the BT hub many smart items would keep connecting and dropping while I was watch their status, some items would not reconnect at all.
My router is the TPLink Archer c2300 which is even sold in the BT shop, fully updated to latest software https://shop.bt.com/products/tp-link-archer-ac2300-dual-band-wi-fi-router-archer-c2300-FT7D.html
I would of checked the router logs and if ONT was having issues then ask for a replacement ONT. But at end of day you've cancelled your service so not much can be done to look at resolving the disconnection issue.
Indeed, I was hoping for a smooth move back to BT, but decided not to get into sorting this out as BT would not entertain extending the 14 day limit to opt out of the 24 month contract.
Easier to go back to a known solid service which is back up and running 100%
if BT had offered a bit more flexibility with investigating the issue I would have stuck with trying to resolve, but they wanted to send out a tech specialist and add £40 to my first bill which I think is unacceptable for a brand new installation