Instabile 5g and LTE connection with iPhone 15 pro max

When my iPhone 15 pro max looses the mobile connection it comes mostly back only as a „edge“ connection with T-Mobile. A soft reset solves the problem, sometimes, but not always. Until the connection is lost again (in a building, tunnel) - then the process has to start again. Sometimes it needs 4-6 resets. Sometimes it happens that the LTE/5 G .swichtes to edge without an interruption of the mobile network. Apple changed the device, the problem is even worse. Resetting the network settings was without success as well as a reset of the eSIM settings, changing the eSIM and change it to a physical SIM and changing the APN settings. T-Mobile and Apple have solution for the problem.