You're talking about a normal windows or other OS update taking all 300 megabit download, and/or possibly using up the 10 megabit upload in 'acknowledgement' traffic?
If so, you could set the ethernet/network adapter on the machine to limit its autonegotiation to only 10/100 instead of 10/100/1000 and your downloads will slow down a good bit, which should also free up the upload a bit. If you ever need to download something extra fast later, just put it back to full auto.
Sounds like it might be a device on wifi though, if that is the case, you can have it limited to 2.4 ghz if you want, or like you seem to have done, change the AP to a router mode and use a fair queuing control that should help manage things a bit more automatically, though it sounds like you might now have double routers which can cause double NAT. That sometimes causes issues of its own.
If so, you could set the ethernet/network adapter on the machine to limit its autonegotiation to only 10/100 instead of 10/100/1000 and your downloads will slow down a good bit, which should also free up the upload a bit. If you ever need to download something extra fast later, just put it back to full auto.
Sounds like it might be a device on wifi though, if that is the case, you can have it limited to 2.4 ghz if you want, or like you seem to have done, change the AP to a router mode and use a fair queuing control that should help manage things a bit more automatically, though it sounds like you might now have double routers which can cause double NAT. That sometimes causes issues of its own.