You are probably better off getting a newer model Surface tablet. By experience, these machines do not have a long life to them.
As for not connection to Bluetooth with the iPhone X; since your tablet will not hold a charge, you have a defect on the machine which could be the issue causing interference with the Bluetooth connection. This defect could be the power management system or the batteries themselves have died. Changing the cord would have done nothing to fix that and the Microsoft Store should have recognized that right off the bat.
There is (or was) a LTE model of the newest Surface Pro which you could probably check out at the online Microsoft Store here: https://www.microsoft.com/en-us/p/surface-pro/8nkt9wttrbjk/h3cs?icid=homepage\_hero1\_ILABv1SurfacePro\_092318&activetab=pivot%3aoverviewtab
This way, you do not have to rely on a mobile hotspot.