Hello,
Indeed, it's possible to provide Helium miners with internet using RUT240 routers. The best way to do it is, like you mentioned, to obtain a SIM card with static IP address and then port forward the necessary port (default port: 44158, TCP) to the miner. If you were to ask your cellular company for a SIM card with static IP address I'm sure they'd let you know if they can provide you with it.
Instructions on how to create a port forward using RUT240 router can be found here:
https://wiki.teltonika-networks.com/view/RUT240_Firewall#New_Port_Forward_Rule
In the configuration page you will need to provide a value for external port (44158) as well as port forward that same port to your LAN device (Helium miner IP). I highly recommend reserving the Helium miner LAN IP address to make sure its IP address remains consistent.
If you have any further questions please let me know and I'll try my best to assist you!
TP.