I give up. Need some help

Ok I’ve tried lots of Google, lots of trial and error and I can’t seem to figure this out.

I’m pretty new to computers and networking. I feel like I had the basics down until I swapped my ISP router for OPNSense on an old OptiPlex.

To start…the internet works great. I’ve got a 2 port Intel NIC and was able to set up WAN and LAN as well as configure unbound and DNSCrypt to work with a recursive DNS server that I run.

The mobo on the PC has a Ethernet port on it too. I went ahead and added it as opt1 when doing my initial set up. I live with someone who visits sketchy websites and uses lots of TikTok, etc. - while it may seem dumb, I do not want the stuff on my network to be able to communicate at all with the other person’s network. I was under the impression that I could use the mobo nic to set up another “LAN” and connect the ISP router to it. I’ve tried just about everything and can’t get it to work. DHCP lease is good and “sees” the router when plugged in but I can’t get the router to connect. I did set routes to allow all on that interface for testing and still couldn’t get it to work. Will this set up even work or should I do something different?

My other thoughts were to just grab a managed switch and work with VLAN’s or grab a 4 port intel NIC because maybe there is a problem with the one on the mobo. Idk anymore and I almost don’t care lol. Am I doing something wrong? I’m new to this so is there a better way of going about this?