Hi, RUT950 has one LTE module so it makes sense for one sim card to be active. Moreover you don't need both sim cards to be active for failover to work. The primary SIM card is the one which is active by default, while the secondary card stays inactive until switched to or set as primary so this is a normal behaviour.
Go to Network > Mobile > SIM Management tab.
Mark "Enable automatic SIM switching", and then choose on what conditions it will activate from SIM1 to SIM2 and visa versa (2 separate Tabs).
Check this page for more information.