I tried 2 different SIM cards from the Belgian operator Telenet in the FLX1s.
Both cards connect to the network for voice calls and SMS. , but no data connection.
Both cards do provide data connection directly in an Android phone after login.
On the FLX1s:
Connected to Telenet
Mobile Network is enabled.
Mobile Data is enabled
Data Roaming is enabled
All bands are enabled
IP configuration either IPv4 or IPv6 does not make a difference.
Network is set to Automatic and it picks Telenet.
Switching off Automatic the device starts looking for networks, but does not find any and reverts to Automatic itself after a timeout and picks Telenet.