iphone 15 pro signal decrese

Go to Settings/Cellular/Cellular Data Options and set it to 5G Auto if it isn’t already. That way it will go back to LTE if the 5G signal is too weak. You can also try LTE only, and see if you get a better connection in some locations.

Signal bars are worthless as a measure if signal strength. The same signal strength bars will vary from model to model, and even between different phones of the same model.

It also depends on what your previous phone was, and what networks it supported. If it was a phone that did not support 5G, for example, then the signal strength of a 5G signal will almost always be different than the signal strength of a 4G/LTE signal, because they are generally broadcast from different towers, and 5G sites are not as densely located as LTE sites because the network buildout is still in progress.

What was your previous phone, and how are you measuring the signal decrease?