You likely will not be able to do what you want. First you need a different router that will allow you to attach to each modem. Mostly these are called dual wan routers. Now you get into the issue you can't solve without the ISP help. You cannot "combine" them to make a single session larger. You can use them as primary and backup or you can manually define which user connection uses which ISP. Some router have the ability to load balance the users to a point.
The only way to actually combine the connection is to work with your ISP. It needs to be combined using similar technology on each end. Normally people asking this question want to use different ISP and that is close to impossible.