A few months ago a friend went onto BT Ultrafast (G.Fast) but was unable to blag a free MT992 modem from the Openreach engineer.
Apparently ISPs that still need them (such as Talk Talk) now have to order them as part of the install & it's a chargeable item.
He didn't like the BT router (or the silly money being asked for a MT992 on fleabay) & other options such as the VIGOR 166 were also pricey.
So he asked me if I could source him a cheaper modem to use with his own TP-Link router.
The answer (after some research) was yes, the BT BUSINESS Smart Hub 2. (MUST BE THE BUSINESS VERSION!)
The firmware in this version has the magic bridge mode switch (in the advanced broadband menu) which turns it into a modem.
I was asked to supply another recently, they sell for around £20-£30 used / £40-£50 new on fleabay, a fraction of the going rate for a MT992
As an experiment, before handing it over, I plugged it into a Talk Talk 150 G.Fast line & it worked fine, so they're not locked to BT when used as a modem.