IPhone 12 dropping 5G and LTE

I have a Iphone 12 Pro Max. Each time when I go outside the house, or traveling between home and work, the data coverage drops and I can't even send imassge out. I suspect it might have something to do with the fact that the device is confused between 5G and LTE. Once it stuck in that mode, no matter what I do (switch airplane mode on and off, data on/off...), nothing helps. I can't connect to any data. The only way to make it work is to reboot the phone. I ended up rebooting my phone three of four times a day, in order to force the phone to work again. Can you please help? Under Cellular, I have roaming off, Voice and Data set to "5G On", and Data Mode set to "Allow More Data on 5G". Thanks.