Erasing an iPhone with MDM

I bought a second hand iPhone from a reseller as my second phone (iPhone SE 2016). Loved the phone but for various reasons I tried to erase it and start again when I got to the script hello message and selected my country etc, it tells me it's subject to mobile device management. I can't get any further.

I'm not trying to remove MDM, but I want to send the phone back for a refund, so my question is:

If I clicked on the erase function in settings and the phone does its thing and restarts with a scripty 'hello' and asks for country etc, is the phone successfully erased? I want to make sure my data is gone before I send it back basically.

Thanks