Welcome to the community!
Thanks for raising this to us, if the issue only occur after the short power cut it is possible that either one of them are defective. For testing purposes, we recommend to connect a different device like laptop/desktop via LAN/Ethernet cable on the modem/router's LAN port in the event that it works fine it could be the LAN port of the XBOX is faulty or you also have the option to try different Ethernet Cable.
But if other devices connected via LAN to the modem are still not working, we can arrange one of our Technicians to check the modem for real-time test and investigation if this is the same, drop me a private message with your account details (Username/Customer ID together with the address on file).
I have tried everything I have read on here about the subject aside from buying a new router as when I first started with TPG NBN the Xbox worked with a LAN connection but now has issues after a short power cut.