FTTB router/modem configuration question
I’ve got an NBN FTTB connection and have been having issues with some devices suddenly having slow internet speeds while others continue to function as normal. Resetting the router fixes this temporarily but every couple of days the problem happens again. I’m still using the ISP connected modem-router so figured this could be the problem. I’ve bought an ASUS RT-AX53U but have just now opened the box and realised this is only a router, not a modem-router. I know very little about how all of this works so could anyone advise the best way forward from here? Ideally I’d love a solution which uses this new router as I’m not sure I can get a refund - any advice would be very gratefully received!
EDIT: should say, the router is just plugged into the wall here, there’s no box in between (which I have had in previous apartments)