If the hub stays running while the internet it out it might be giving up on reconnecting after a while. I am not sure how that works exactly.
The Hub Info Driver v3 has a option you can setup on it that checks the DNS and/or a cloud URL periodically. You could use that to then trigger a hub reboot if it fails for a certian time limit.
Another thing you could do is get a power monitor plug [DRIVER] Zooz ZAC38 Range Extender (and Power Outage Monitor), and when power is restored then trigger a hub reboot like 5 minutes later, that will force it to reconnect.
