Regarding the mobile connection failure, I recommend you check the APN and operating band configuration. For more information on these settings, I leave some links below.
Operation band: https://wiki.teltonika-networks.com/view/RUT240_Mobile#SIM_card_settings
APN configuration: https://wiki.teltonika-networks.com/view/RUT240_WAN#Mobile
Regarding the possibility of remote access via RMS. It is possible to connect the router to an ISP through its WAN interface and connect to it remotely through RMS. For this, you must first create an account in RMS. Then you must register the device on the RMS platform. After registering the router on the RMS platform, you can log in to manage it as long as it is connected to the Internet. You can access it via CLI, WebUI to check configurations and view logs of what is happening on the router. For RMS, after the first month, you will need credits to continue accessing the device. Here are some interesting links about the RMS platform, with this you can perform the task you want.
Create RMS account: https://wiki.teltonika-networks.com/view/How_can_I_create_an_RMS_account%3F
Add device to RMS: https://wiki.teltonika-networks.com/view/How_to_add_devices_to_RMS%3F
Remote ssh connect: https://wiki.teltonika-networks.com/view/RMS_Remote_SSH
For more information, I leave you this link to the RMS information page.
Thanks for your response.
my focus is on as you say "It is possible to connect the router to an ISP through its WAN interface", I need to know what configuration is necessary for this to work.