If you haven't got a new box yet, I'll also add that I just tested recently a TP-Link AC4000 WiFi 5 router, connected to an old BT OR (HG612) modem using PPoE auth and IGMP enabled on the AC4000 - works perfectly even with UHD/4k BT Sport channel.
My only gripe is that I have a weak WiFi (with all routers) at the back of the house and this is not a mesh router so it is going back with much sadness. The BT complete WiFi discs worked pretty well but I need to split the 2.4/5GHz SSIDs out which you can't with the BT kit.
However I'm a little concerned now as I have just ordered a new Asus Mesh (AC3000 CT8) which does have IGMP capabilities but less user options! Apparently Asus has implemented a new IGMP daemon (mcpd) which has IGMP snooping capabilities but all config is set when it detects the WAN! Lets see what happens tomorrow!!
I have purchased a D-Link router with a Draytek modem. BT sport still doesn’t work but I am going through the motions 🙁. It is still better than the Home Hub 6 for WiFi coverage by far, but at present it looks like every couple of weekends I will have to use the HH6 to watch the Premiership rugby. Not a great advert for BT and their tech support TBH.
Asus tech support only seemed to get interested in the fact that I was having problems with their product when I pointed out that despite contacting BT tech support, the BT community, and Asus tech support there was no record of anyone having an AC88U router and having successfully made it work with BT sport. By this time the router had been returned.
I tried the Draytek 130 modem and it was 95% OK for me. After configuring to emulate the BT OR modem (for TV channels) it worked fine with my TP-Link router for BT TV channels EXCEPT BT Sport UHD/4k ..... it just pixellated all of the time so that went back as well!
Problem solved - I have bought an Amazon Firestick to access BT Sport.
I couldn't fix the problem with the Draytek modem and D-Link router, and the situation was still unresolved with the replacement TP-Link AX6000. However, the router has great coverage so I have kept it.
Fortunately, BT Sport now have access to their channels, including 4K UHD, via a Firestick, Apple TV and other devices so I have given up on configuring a modem/router and spent extra money on a means to access the sports channels on my TV.
My suggestion to anyone seeking to use an alternative router to that provided by BT is to plan to spend the extra money on a device that will give access to BT Sport via an app, and focus just on getting a wifi solution that works really well for your home. Then, if the hassle of trying to get BT Sport to work on your new modem/router/wifi extenders gets too much, just give up on the BT box and buy a Firestick/Apple TV/other device. Do not expect any tech support from BT or your modem/router company to be any use - that way you won't be disappointed.
Many thanks to all who contributed to try to find a solution to this problem.
I ended up with the Deco M9+ 3 unit system, pretty good house coverage and its working 99% with BT Sport UHD/4k
BTOR Modem >> Master Deco M9+
| ----- Deco 2 (wifi backhaul to master and phys eth to BT YV box)
| ----- Deco 3 (wifi to Deco 2)
Keeping my eye on it but touch wood, pretty good so far