I am assuming you can't find the Settings on the Stick to find and change the new Router?
If that's the case from the NOW home screen on the Stick, keep pressing the back button until you see the additional menu down the left hand side of the television screen where you should see Settings, then go into Network.
Should you have already found this, then double check your Router is not automatically pushing out a high channel wifi number.
If on 2.4ghz select somewhere between 1 to 11 or on 5ghz somewhere between 36 to 48.
Heres a tip that will save some time.
If you have a new router instead of changing all the countless devices to the new password/wifi name - and you will probably have more than you think, just change the new routers wifi name and password to the same password you had before.
That way you only change one device as opposed to countless others, and if like some of the stuff i have the whole faff of re-configuiring the wifi with awkward apps etc will be avoided.