Im not sure if mine was an accidental fix, but i was trying multiple things at the time so i cant be sure if this will work. but from what i gather its an xbox issue not the router.
i was trying to set up a static IP on my xbox https://www.youtube.com/watch?v=bVInhZWK_pQ (video tutorial on static IP) anyway it didn't seem to work for me but what i found was after id switched to manual IP and DNS to set it up and then subsequently changed it back to automatic IP and automatic DNS on xbox settings it seemed to connect, iv been connected perfectly ever since.
i hope this helps, follow the static IP tutorial as that might work for you, but if not then just do as i did and swap it back to Automatic. have a play around and sometimes the answer finds itself lol
good luck
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