'but since I own the modem now, they wont replace it for free."
So your NOT paying anything for the modem for on your bill then.. So its not like you leased it, you just bought it from them on a payment plan.
If they are still charging you a modem lease fee, then they should replace it with their current modem if there is something not working on yours.
If your modem is suppose to be docsis 3, but your only getting connected at docsis 2 - then seems more like an isp issue and not a modem issue. Purchasing your own docsis 3 modem would not fix an issue on their end.
Where exactly are you seeing your only connected via 2? Could you post what your seeing that makes you believe your only on docsis 2 vs 3?
I personally don't like the lease option on any sort of hardware - I will just buy my own thank you
I have a SB6120 and have not had issue one with it, on comcast. But if they were just charging for it, and then stopped - that just sounds like some sort of rent to own thing, not a normal cable modem lease deal.