This forum is for support of the OBi device. If you're trying to get a more stable internet connection, you need to start with your cable company. If your internet is slow, your provider needs to diagnose why and either a) get you to pay for higher speed or b) figure out why they aren't delivering the speed you're paying for.
Unplugging SP2 from the OBi device should have NO IMPACT on anything, except that you can't use SP2.
You can't just add a different cable modem to your service. Cable modems have to be provisioned in some way by the cable company, i.e. on their end they tell the system that cable modem xyz with id 123 is now your cable modem, and set it to deliver speed of xxxmbps. If you buy a cable modem they don't approve, you bought a paperweight.
You can use any wireless router you want. Yours should be fine. However, you need to diagnose where you're losing connection - the cable modem or the wireless router. If it's the router, it could be channel interference (for wireless) or something else. One way to test that is to see if a WIRED computer connection is lost when you "lose" your connection. If not, then it's a wireless issue which could be a bunch of things. If it loses a wired connection, it could be other things.
My advice - focus on one thing at a time. If your internet is slow, it has nothing to do with your OBi. Get that resolved with your cable company. Then, when it's resolved, you can focus on whether you're losing internet connection wirelessly, wired, or what. Then you can focus on your phone service.