No sync when another BT Wholesale connection at the same premises continues to work implies a failure in the MSAN line card port, your router, or the metallic pair in between the two. The cause could be technological or human error (mistaken cease, accidental or malicious tampering with the line).
It's probably worth ringing 17070 on the affected line and checking that you hear the phone number you expect, also that a quiet line test is OK. It's worth trying the test socket, a different cable between the socket and the router, and a spare router if you have one. If none of those work, it's a case of calling Zen.
My Zen connection has been working fine all day, as you can see in my BQM, though that means nothing in this context as it's a FTTC connection via the Zen backhaul network.