I've got the same issue. I just came back to BT a couple of days ago, my line has been working with SKY for 12 months at 40M speed, not one single drop in the profile since I joined.
I know in my line I can achieve up to 56M-60M as I had 76M profile before with BT.
1st day I start up the Router, it connects, speed 55M synchronized, and 7ms ping, awesome.
2nd day, without touching the router it reboots, speed dropped to 52M, 10ms ping. OK not a problem.
3rd day, router reboots again at same time the day after, line gets to 49M, 15ms ping.....well that's not what I expected. This is the minimum of the advertise speed by BT on my line. Remains stable for 48 hours no drops.
I reboot manually the router. Line gets again back to 55M. I used the line yesterday evening without any drops, and no errors in line.
Overnight the router had been reboot again by BT side and synchronized again at 49M.....LOL. It's a lost battle.
Thanks for your support.
I decided to put the HG612 in place together with an RT-AC68U I've got. Thanks to this I found out I was having a lot of ES/CRC, so I tried to find out why.
It turns out that I had a Home Plug close to the VDSL socket, and VDSL2 and Home Plugs close are not a good choice. I was losing when the Home Plugs were in use, between 1.5-2dB of SNMR, thus the increase of ES and the retrains.
I disconnected all the Home Plugs and left the HG612 and the router plugged and it synched at 62MM sharp, which is the Max. my line can attain. My packet is the 52M so obviously, there is a cap.
I plugged the Home Plugs again at least 2m far from the modem and the socket, but still the PLCs will produce some noise that would affect the line. Right now I'm showing 59M max. attainable and 54M synched, although when testing the max throughput I can get is 50-51M, which is almost the "ideal" of the line.
However there is a but. When I first connected the router, I was getting G.INP on my line, so no delay and no Interleave, however now that the line is stable, it's being correct and no errors, the G.INP is not coming back and the delay is still there. I'm on a Huawei exchange, so this should be coming back.
Do you know how long can it take? Anything I can do to speed up the DLM process? Thanks for the support.
PS: Don't plug your Home Plugs close to your router/modem. If possible don't use Home Plugs at all (if you have VDSL)