The only two suggestions I would recommend are:
Keep Airplane mode ON while you’re at the place where there’s *No Service so you exclude the phone for constantly using power by searching for a signal it will not pick up until you get outdoors or another area where a cell signal is available.
Once again, keep the phone on Airplane mode, activate Wi-Fi and Wi-Fi Calling if the are where there’s No Service, a Wi-Fi hotspot is available. That way your phone will have basically a full cell service through Wi-Fi excluding the wasting battery power on keeping the Cell Radios ON.
Make sure, if you use the second recommendation to not walk out of range of the Wi-Fi hotspot while on a Call and having the Cell Radios OFF from Airplane mode to avoid the Call to drop. Make sure, if you’re leaving the area to tell the person you’re speaking with that you’ll call them right back. Turn OFF Airplane mode on your way outdoors and you can then proceed with your call.
Edit: there’s a third recommendation as well to mitigate this issue and that is to get a Free 4G LTE CellSpot from T-Mobile and install it in the area that there’s “No Service available using an ISP’s connection.
Others can chime in with further details on how you can get that set up and running.