Unfortunately, with your current set-up, only the "Failover" feature will allow you to back up your primary WAN connection in case it goes down.
Take a look at this link for reference: https://wiki.teltonika-networks.com/view/RUTX11_Failover
It is not possible to interchange the settings between "SIM Switch" and "Failover" features, so you will not be able to use such settings as: "on SMS limit", "on roaming", etc. with the "Failover" feature.
If you would like your WAN connection to switch whenever you reach a set Mobile Data limit on your SIM card, you can set up the "Mobile Data Limit" feature in the "Interfaces" page. Navigate to Network → Interfaces, find your Mobile interface (it should be called: mob1s1a1) and click on the "Edit" icon to open up the settings page. Navigate to Advanced Settings → Mobile Data Limit and enable the "Enable data connection limit" option.
Take a look at this link for a more in-depth explanation of the available settings for this feature: https://wiki.teltonika-networks.com/view/RUTX11_Interfaces#Mobile_Data_Limit