Can't erase data from an old iphone 6

I'm trying to delete all the data from an old iphone 6 (which I haven't used for a year), but when I try to erase all data, the phone tells me that it can't complete the request because it can't connect to my apple icloud account. I don't have a SIM chip in the phone at the moment, and I'm wary of putting in my current sim card (or connecting the phone to the internet), because I don't want the old phone to create any confusion with my current iphone.

How do I delete the data, when my iphone won't let me?