I have similar issues.
I have been on a FTTC (BT Infinity but with different ISP) for a few years and am stuck with a fairly slow (20-25mbps DL) due to my location.
That said the service has been pretty reliable and the speed pretty stable over several years.
I have just "upgraded" my old OpenReach modem as my ISP advised they were no longer "supported" (whatever that means in practice). I have a swanky router (ASUS RT-AC88U) so I thought I would get a new modem to match.
It took a lot of pain to get it to sync at all and I could not get it to work until I updated the firmware to 3.8.4.1 after which it synced straight away.
But now my download speed has dropped from 20-25mbps to about 15-16mbps!
If I'm honest, perhaps I was hoping the new modem would result is a slightly faster connection but in fact the opposite seems to be the case.
Maybe the line needs some time to stabilise? Due to the hassle I had getting it to recognise the PPPoE service I had to reboot both modem and router multiple times over a couple of days, so maybe the line "thinks" there is instability there?
I think I may revert to the BT modem if things don't improve.

Sounds like the DSLAM may be applying corrective measures to stabilise the line. It really depends on how many times you reboot.

You could also try different firmware versions; they'll offer different synch speeds, too, although not as dramatic as you might want them to be to synch you at 20-25Mbps or more.

What's your SNR margin?

Hi Edinburgh, appreciate your input.
Well, this is confusing...
Previously the Draytek Dasboard was reporting about 17mbps down, but now that has gone up to 22mbps.
Modem reports SNR 7dB for both up and down, but Speedtest is still only 16mbps down.
So it looks like the line is stabilising but the speedtest is not yet responding.
Hopefully you can shed some light on this for me, 'cos I'm scratching my head now.

It depends on what speed the 'dashboard' is reporting. The actual rate will differ from that reported by a speedtest checker. My actual rate is 53Mbps, whereas effectively I can 'only' get 45Mbps. So, I'd say 16-17Mbps actual speed would be right with the 22Mbps sync speed.

You can check with the BT Wholesale checker the speeds for your area: https://www.broadbandchecker.btwholesale.com/#/ADSL

Other factors can influence the speed, too, such as the ADSL/VDSL filter quality, modem codes on your Draytek router, WiFi channel etc.

At 7db, you can try and see if the situation improves a bit once you have a bit of stability, especially once the SNR drops to 6db, and potentially to 3db if you're on a Huawei cabinet.

Remember also that different modem codes for Draytek routers can produce a different type of performance.

If you test with the BT speed tester, which I think requires your phone number for the full test, you can see the IP profile speed for your line alongside the sync speed.

If the sync speed has improved, it's possible it will take a few days more for the 'ip profile speed' to also increase, which afaik is managed by the automated DLM system.