i use a very simple workaround for those connection problems:
i never turn on wifi and mobile network at the same time.

you can set shortcuts for both on the settings app and switch easily by hand, when you go outside for a longer time. as long as i'm at home or in the office i only use wifi, when i go outside for a longer time, i turn wifi off and mobile network on.

no problems and very long battery life, when using it that way...