Hello,
What IP address you will have on mobile interface (mobile WAN) determins a ISP. ISP also provides you with the information what settings to use for SIM card in order to connect to mobile operators (for exmaple, what APN to use, what authentication method and etc).
- What type of connection mode (NAT, Bridge, Passthrough) do i have to use?
Depends what you want to achieve, if you want that your end device would receive and IP address from ISP, then you can use Bridge mode or passthrough. If you want only to have ability to access devices remotely which is in the routers LAN network then you can use "port forwarding" on RUT955 -
https://wiki.teltonika.lt/view/Firewall#Port_Forwarding