My hub 3 suddenly decided it doesn't like being a modem. It didn't like being a router either for a little bit but then I got some helpful typing help and then the restart worked properly, but if I try and modemify it , it doesn't like it anymore , this happened at some point in the last fortnight when I foolishly left the house to see relatives...that'll teach me, outside bad, inside good. Any help would be greatly appreciated, thank you.
best option is to perform a full factory reset - depress the reset button (using a paperclip, sim tool or similar) for a full 60 seconds.. don't switch the hub off, leave it be for ~10 mins afterwards for it to fully settle down - then see if it works after that..
if that doesn't work, i'm not sure what other options you have (other than requesting a new one)