Modem/Bridge Mode

I have a older BR1 with 3 ethernet ports as well as a BR1 Mini. When using them solely as an LTE modem, is “IP Passthrough” all I need to check or do I also need to disable the NAT option?

I have been running the Mini with only IP Passthrough, I have the router connected to it using the WAN/LAN port. On the older BR1, which LAN port should feed the external router? (The three ports are labeled WAN/LAN1/LAN2)