My phone was stolen and they immediately turned off find my iphone. How do I erase my phone?

My phone was stolen on Tuesday. When I arrived home I had 3 emails (from email addresses that were 100% genuine apple ones, I checked!), two saying my apple id password had been changed and one saying Find my iphone had been turned off. I never clicked on any of the links in the emails just to be safe, but tried to get into my icloud by googling the page. I had received those emails no more than 5 minutes after I was robbed, but couldn't see them until I got on my Macbook at home. I believe they were able to get my passcode by following me, as with a mask on I couldn't use face ID. But how did they manage to get into my apple ID?!

Anyway, I now have a new phone and finally I am able to access my icloud, however the stolen phone is still showing up there. How do I know if they erased all the data from it, so I can feel safe that they don't have access to all my info?

I can't erase my phone because it isn't showing up on Find my iphone due to the thieves turning it off.

Oh last thing, I know they didn't do anything with my bank details, instagram and facebook accounts etc. because I have checked them every day since it was stolen. So I assume they just logged in, turned off my find my iphone, changed my password and wiped the phone.

But I want to be sure and have no idea how to do that!