Cellular data usage suddenly jumped up

I have two iPhone 10s sharing in a plan. My wife and I use very little data when out of the house. In fact, we usual have the cellular data selector switched off unless we need it fit an immediate moment. Suddenly, with no change in our habits, we’re being told we’re using nearly double what we used to use, and this month we actually pushed past the limit of our plan and will be charged extra. It seems that this correlates with the introduction of 5G. Our phones do not support or use 5G.