Not sure how technitium works but just from my selfhosting experience are you sure your not hitting dns-rebinding protection somwhere.

In short DNS rebinding stops domains from being resolved to private IP ranges so you don’t end up back in your Network when you seem to be resolving a public domain.

I have to set up any domains that resolve locally in my router (which also does DNS and DHCP) but not sure if that’s necessary with technitium

They absolutely do have “Startup Applications” in gnome not sure what the article is describing but I usually use gnome-tweak which offers a simple interface that shows currently running apps and a list of all apps and easily lets you select them to start at boot