I have 6 M4 and 2 M3W used 

in AP mode with ISP Fastweb modemrouter (used as DHCP Server and Gateway, static IP), in a ethernet network with 30devices (15-16 connected phisically in a switch), with some very rare offline of the single deco. 

2 days ago the external dsl cable, from office to the ISP cabinet, became faulty. So i decided of upgrade the deco family with X20-4G and now start the problems! I can't use it as 4g backup mode because the sim is not recognized and the app will show the pop up 'FAIL' and I can't enable the internet mobile data. I have rebooted, changed sim and reset the deco and the entire network. but nothing changed. There's some bug with this feature or the app? the app also deleted the new network (configured starting from x20-4g in AP mode) when I selected and deleted the old network, please help!

The sim was inserted