SNMP variables for RUT955 can be found here: RUT955 SNMP - Teltonika Networks Wiki
As for determining which line is active, we currently do not have any OIDs for Failover, however, OID for interface state is available: .1.3.6.1.4.1.48690.11.2.1.3. It could be used to determine whether the WAN or Mobile interface is active or not.