This can be achieved using VLAN functionality. Firstly, you would need to go to WebUI Network -> VLAN section and create a new VLAN by pressing the add button, then select which port will belong to which VLAN. Let’s say you want the 3’rd LAN port for your second LAN IP 192.168.100.1, then you would select on VLAN ID 3 LAN 3 and Untagged and on VLAN ID 1 you would select LAN3 as Off. That would allow anything connected to LAN 3 port on the router only connect to 192.168.100.0/24 network. There are more VLAN options, that you can read about in our wiki: https://wiki.teltonika-networks.com/view/RUT950_VLAN this implementation is the easiest one.
Next, you would need to create and additional LAN interface in WebUI Network -> Interfaces in the bottom write the name and press add, in the popup windows as protocol select Static, write down the network address off the second LAN interface, based on your post, that would be 192.168.100.1 and mask as 255.255.255,0, then press Setup DHCP server. In physical setting tabs select the interface as eth0.3 where the 3 means the created VLAN ID. After that press Save & Apply at the bottom. You can read more here: https://wiki.teltonika-networks.com/view/RUT950_Interfaces#Add_Interface