ASP Classic web app times out after 2 mins no matter the setting in IIS or web.config

Question

I'm running Classic ASP web application hosted via IIS and everything is set to timeout after 5 minutes.

httpRuntime is set to 300 Server.ScriptTimeout is set to 300

Still a request to generate an excel export which takes more than 2 minutes fails with Gateway Error 502 exactly at the 02:00 minutes.

Looks like there's some setting which has precedence over the above but which?