No internet connection one week after "Go Live" date

We recently switched from BT to Talktalk (last Monday). For the past year or so we've had no issues with our fibre connection.

Since the go-live date we've had no connection. The Internet light is off on the router. Broadband and Wireless lights flash continuously.

We had a Talktalk engineer out who didn't appear to do much. He tested another router which also didn't work and recommended a BT Openreach engineer.

The BT Openreach engineer came out this morning. He tried all sorts of tests and said that our line was syncing fine in his tests. He said the issue was either with the router (which was proved not to be the case by the Talktalk engineer) or something wrong on Talktalk's end.

I have tried all of the usual solutions. Test socket. Resetting router. Etc, etc. The phone line is fine with no noise at all.

When I call customer services now it says that the "engineer is investigating the issue and I will be contacted within 72 hours." I'm really annoyed with his as Christmas is next week and the kids won't be able to use their presents.

Does anyone have any suggestions? Thanks in advance.