I had a similar setup once, and the M9Plus doesn’t automatically switch to a mobile hotspot when the main line drops. Try disconnecting the Ethernet backhaul temporarily so it connects via Wi-Fi from the X50. Once it links up, you can reconnect the cable that usually forces the sync properly, just like how I fixed a similar issue while setting up my c444 network.

  @william512 Thanks for your suggestion. Now the internet provider restored the connection and the network works in the normal way. Just before this, I was thinking about using a CAT6 lan cable to connect the modem to the WAN port of the X50, doing a manual switch.

If I understood well, the WAN backhaul prevents my three M9plus from connecting. I imagined that the backup hotspot substituted the WAN in a transparent way; If I have to do a manual disconnection of them, I prefer to connect the 5g modem by its LAN port

Anyway, I would suggest to the Tp-link engineering to evaluate this as a missing feature when they implemented the connection by an hotspot; the alternate internet connection, once enabled, should be routed not only in the main unit, but also in the satellite one, on all the topology of the network, otherwise it is not so useful.

Emanuele

I also realized that when the X50 was in  "hotspot backup mode", the system time did not propagate from the X50 to the M9plus; I saved a screenshot

After the connection on the WAN port was available again, the system time propagated to the M9plus, now it is:

As I wrote before, I would suggest an evaluation to the tp-link support team. The feature they added, the hotspot backup, is very useful for people who study and work from home and have a critical need of connectivity.

Emanuele