Never had a problem either

You have to get a whole workspace thing and administer your email like a company. And there are many settings to get wrong! And problems like this where some Google products don't work or are limited.

Ah, you're talking about a custom domain to use with GMail specifically. Yes, for that either you use "Email forwarding" if you got your domain from Google Domains, which lets you receive and send emails but the sent emails are not SPF valid, or you use a Workspace. I used to do the "Email forwarding" thing in the beginning but then I moved to Fastmail.

I got G Suite or whatever it’s called now before it was paid and before Google Domains sadly. I discovered unbelievably late that I didn’t have two factor on that account because for some reason the G Suite admin panel was defaulted to disabling it organization wide (aka just me). I don’t know why they don’t just have a $20/mo option in gmail that just does this properly in one click.