Try dialing on your dialpad *#*#4636#*#* and go into the phone information menu. If NR state is "NONE" and NR available is "false" in this menu, that means your phone shouldn't be using 5G, and it may be that your network provider is incorrectly charging you. To be extra sure, you can set your preferred network type to LTE only.
If this doesn't work, NetMonster is a third party app that will give you similar information, but I believe it's only officially on the Play Store. Network Survey on F-Droid will also give you info about the tower you're connected to.