I think a statement something like this would have given me deeper insight into
how to resolve the problem:

If mobile broadband (3G/4G) does not work, while all other functions (wifi, text
messages, phone, GPS, installed apps, clock, etc.) otherwise work as normal, and
you have the same problem in different locations, so that the problem is
unlikely to be a service provider fault, then it is likely that you do not have
a hardware fault requiring a repair, but do have an option set wrong. First of
all use one of the options to save/back-up the contents of your device on the
internet or a local machine.

Then, in order to identify and remove the problem you may have to check all the
permission settings within android and preferences in particular apps. If in
doubt restore them all to defaults, one at a time, until you find that you can
connect again. If this does not work, try removing apps one at a time or a few
at a time and rebooting after each removal, perhaps starting with apps that you
have used recently.

As a last resort you can follow online instructions for sending the machine for
repair, after backing everything up and deleting all private information.

(I am not an Android expert. There may be a better way of drawing attention to
most likely settings to change.)