I'm looking for a piece of hardware to connect to different types of ISPs, normal DSL, cable DSL and USB egde sturfstick to offer one Internet connection with a higher cumulated bandwidth. I'm pretty sure something like this exists, but I don't know how to call it or where I can find something like that.
Something like the Peplink Pepwave Max Mobile Router would accomplish what you're looking for.
If you want a cheap router to do it, you can buy a DD-WRT compatible one (I'm running an ASUS RT-N16 in the office, it's great!) and config it yourself