You still use the carrier sim card for a connection. That doesn’t depend on what OS you’re using.

The tankless heater should have a map in the specs divided into zones by average water temperature. There’ll then be a table that references the zones and what size you need for what you expect to heat at the same time.

You could use a nail set so you’re not denting up the drywall.

I’d also recommend using a screw to actually hold the drywall in place, since that nail is just going to work out again. But if the nail isn’t out enough to easily grab without denting up the drywall, using a nail set to drive the nail back below the drywall will do less damage than other methods.