Bernd,
It can be any number of things; the information you provided is not sufficient to determine the problem.
You didn't specify if the mobile router is on the control or radio side. It generally can not work on the radio side,
but I can provide details if that is the case.
Otherwise, you are welcome to mail me at dj0qn (at) darc.de and I can send you my networking checklist that may
help you locate the problem.
73,
Mitch DJ0QN / K7DX