I've been using the Android mobile hotspot to get an Internet connection while traveling. The stock hotspot is fine for my laptop which is firewalled, but there are a few devices (smart TV, game system, etc) which are not at all secure to put onto an open network.
I want to have a firewall on my mobile hotspot (for blocking all external traffic except return traffic). Is there an app for that? Preferably open source but even paid is fine. My Android isn't rooted, so I would prefer a solution that doesn't require rooting my phone (if possible).
Just in case you might think NAT provides adequate firewalling for security, read here. TLDR; NAT is not a security layer.
I've already tried searching for this here, but I only found a question from someone trying to filter IP addresses that was never answered. None of the hotspots I found in the Play store offered any port blocking features.
I should mention that I have reason to believe my carrier's NAT does not block incoming connections. That reason being that running a web server on my phone using NodeJS in Termux will be accessible over the Internet.