I bought an AX72 to replace the Sky Q router for my gigabit FTTP connection. It works for the most part, I no longer need the Sky Q router and connect the AX72 directly to the Fibre ONT.

However, I get problems whenever I do any sort of serious downloading that maxes out the speed of my connection. For example I downloaded Fifa 23 from the EA app this morning in about 5 minutes, and after this I was unable to load a number of websites eg bbc, Epic games, however others would work just fine. I rebooted the router and then things returned to normal. So I deleted Fifa 23 and downloaded it again and bang, same thing, websites stopped working.

They resolve in DNS

It affects all machines on the network (wifi and hard wired)

I've tried MAC cloning my current PC, and changing the MAC to be the same on the Sky router

I've tried changing DNS on the router, my PC, in DHCP, it has no effect.

If I put my Sky Q router back in between the ONT and the AX72 this problem does not seem to occur (yet anyway). 

Any ideas? I'd rather not leave the Sky Q router in place because the bufferbloat is horrible on it.