[Q] Upgrade modem? topic

Hi. I'm currently on CM12 and I have been facing this problem(?) for a while now. Back when I was on stock 4.4.2, most of the time my phone is stuck on EDGE when I'm using my primary sim card (network A). In order to get 3G/4G signal, I have to explicitly choose a network operator. After a while, it reverts back to EDGE so I have to do that again. However, using my other sim (network B) doesn't have this problem. Now that I'm on CM12, I'm still facing the same issue. I can't use that fix since I think there's a problem with the rom itself when choosing the network. I'm thinking either this is a network error or a problem with the modem itself. I'm currently on NF1 and I'm wondering how to upgrade my modem to the latest one. Thanks!