Seller hasn't responded what should I do?

I bought a iPhone 6. in the description it specified that it was fully functional, that it could only have some scratches on the outside but nothing when it came to if it functioned, it arrived looked pretty good but everytime I connect it it says 90% then I disconect it and it says 10% or directly turns off, and when i can manage to keep it on for a few minutes it starts restarting itself, i tried contacting the seller as he also specified to pls contact him before opening a request with ebay. but its been 3 days and no answer, should I open a ticket or wait?? its my first time buying electronics in ebay🥲