Iphone x battery

hello i have recently got an iphone x which was in use already, the guy when sold it to me have shown everything tha battery health was 89% and it was working fine. Now after almost a month the phone has started crashing it shuts down and then restart itself with the battery level 1% and at the same time when i check the battery health it says Service.

After a while it gets back to the normal battery level and again the battery health gets back to 89%. Im so much confused and worried and dont know what to do.

There are days when it works absolutely fine and never shuts down but then there are days when it restarts 6-7 times a day.

Please suggest what should i do i have already done the Reset, Reboot, restore iphone etc