A bit of a long shot but you could always power down the router and take the PSU out of the socket and then after ten minutes reconnect everything and power up the router.
After that you might find the connection running through another gateway too.
That's virtually how I tackled it. Just switched off everything for ten minutes. Seems to have cleared it, but I hate things like that, where there's no scientific explanation.
I didn't remember to check which gateway I was on before I powered down, unfortunately, so don't know if I've got a different one now or not. It certainly seems like something has changed.