Keyword is "fanless firewall" The following devices are designed to be used to build your own router: https://www.pcengines.ch/apu2.htm https://protectli.com/product-comparison/ http://www.qotom.net/product/list-58.html https://www.amazon.com/GL-iNet-GL-MV1000-Computing-pre-Installed-Performance/dp/B07ZJD5BZY https://www.amazon.co.uk/Turris-hi-performance-printserver-virtual-dual-core/dp/B07XCKK146
Resellers: https://www.pondesk.com/ https://linitx.com/
Big chain resellers: https://www.aliexpress.com/popular/firewall-fanless.html https://www.amazon.co.uk/s?k=fanless+firewall&i=computers
NixOS on firewall devices: https://www.reddit.com/r/NixOS/comments/5biazh/nixos_on_pc_engines_apu2/d9pyubc/
CPU can be a bottleneck: https://www.amazon.com/gp/customer-reviews/R2L78EIVV8NICZ/
Same question as I have: https://www.reddit.com/r/homelab/comments/9ucsvu/question_looking_for_a_modern_fanless_firewall/