Don't have a gateway to test with, but try logging into the gateway at 192.168.0.1 > user:admin password:password > Parental Control > Managed Devices. Click on the X to the right of the blocked device to unblock it. From this thread. Probably a way with the app too but can't find instructions for that.
I have no idea how to find how to log into my gateway as the technician set up my router for me. Been searching for two hours to figure out how to reconnect my blocked devices app and internet with no luck.