TLS (websites) on your personal device is not being MITM by the Fortigate. TLS on your school's devices are. The page you're not seeing with the certificate er

TLS (websites) on your personal device is not being MITM by the Fortigate. TLS on your school's devices are. The page you're not seeing with the certificate error is just the "This site is blocked by FortiGuard" page. You're not seeing it because you don't have their cert installed in your device (which is good, don't install it). If you visit a blocked site on a school device, you'll see the page. Sorry, I don't know how the Forti's block DoH. To everyone recommending VPNs, SSH, etc: the Fortigate does application control and is going to catch like 99% of those shenanigans. Side note: Bypassing the network guardrails setup by a school or workplace is usually against their usage agreement. Even if you're not bypassing for nefarious reasons, it's probably not worth the potential consequences of getting caught.