Can I connect my Time Capsule wirelessly to my mobile router?
I'm afraid you can't.
Extending networks only works between Apple products and the Time Capsule does not have a "client mode", as the Airport Express. You could buy a router that can connect wirelessly to your ISP Modem - Router, and connect the TC by an ethernet cable to it, in order to keep using it as a Time Machine destination. You would have to put the TC in bridge mode. An Airport Express will be able to fulfill this role.