Hello,
Scenario 1
Yes, it's possible to assign a different IP address to each LAN port. You can reach the internet via the mobile interface when the computer has 10.10.10.8, 192.168.20.30, or 10.50.50.60. Also, you can use SMS gateway, HTTP post/get functionality and you can connect to router via SSH from any LAN port.
Scenario 2
Yes, it's possible to assign a different network to each LAN port. Computers connected to a LAN with DHCP off will not be able to access the Internet. Computers on different LANs can send data to each other. This requires to configure zone forwarding in Firewall. Also, you can use SMS gateway, HTTP post/get functionality and you can connect to router via SSH from any LAN port.
You can configure via WebUI and CLI.
Some information you can found here:
https://community.teltonika-networks.com/19988/two-dhcp-servers-with-different-subnets