Please note that these instructions are for RUT240 with firmware 7.0 or higher.
You could try to enable the "failover" function on a single interface that has the SIM card inserted and provides your device with internet connectivity (mob1s1a1 for example) and then configure it (or leave default settings) to ping any reliable host, for example Google DNS server 220.127.116.11, every few seconds. If you lose connectivity, the interface will be forced to "reload" itself by putting it to "down" state and then bringing it back up a few moments later - this might be enough to bring your internet connection back.
To do this, simply navigate to Network>Failover and enable the mobile interface. Make sure to save & apply the settings and see whether this setting solves your issue in the long run. Please note that if you have a limited mobile data plan, I'd recommend editing the default settings and setting the default interval to a higher (longer) value.
I'm also leaving a wiki link for you for more information about failover here: https://wiki.teltonika-networks.com/view/RUT240_Failover
Let me know if this works for you.