My idea is to use WAN without internet access, and Mobile connection only optionally for internet (Internet connection is accessible on the third network , not direct connected to my networks), and it will be switched on in the case of main internet line failure, for instance by telephone call.
I have tried some settings according Your text, but i could´nt get it to run. Probably the descriptions in manual are not quite clear for me, and I am not very good at network routing. Would it be possible to describe more precisely the settings that You indicated in the previous answer?
Thank for help,