So i am moving soon, and will be setting up a new LAN. A bit of an unorthodox one. Internet in the city im going to school in is pretty expensive so I've got an old thinkpad running OPNsense and will use that as my router and just tether my phone to it to provide cellular WAN. Both my PC and Laptop use Linux, Mint on PC, Debian on Laptop. I'm wondering what steps i should be taking security-wise on a setup like this? All my traffic will be going over the open air, and i know highjacking cellular connections is something thats done. Other than just using a VPN all the time which im doing are there any other steps i should take, maybe in my computer, or router firewall, that could help prevent MITM attacks? I'm not super familiar with how router level firewall especially works. Is much tweaking even required?

technically OPNsense is FreeBSD based not Linux but hopefully people still know a bit about it