Unable to sign out and erase iPhone SE for trade-in due to unknown error

I'm desperately trying to sign out of my old iphone SE so I can trade it in, but I seem to be stuck in a loop. The problem is exacerbated by the fact that there is only 1% battery on the SE so it keeps cutting out. When I do get to the sign out screen to erase all content etc. and put in my password, it shows a message "An unknown error occurred. Please try again later" which I have done on numerous occasions - to no avail! I have removed Find My Phone on the SE (which said it would be effective yesterday) and I've tried to sign out of the SE using my new iphone but then I get various messages like "Iphone isn't at a familiar location. To erase all content and settings you first need to turn off Find My Phone"!! When I look at the Find My Phone screen on the SE, it is sort of greyed out so I can't click on Find My Phone on there to turn it off although I've had an email from Apple to say that FMP on the SE has been disabled. I went into my Icloud account using my PC and the SE phone is not showing on there now, presumably because FMP has been disabled. I am tearing my hair out trying to solve this and would be SO grateful for help on this. Thank you!