I'm not going to be able to help much because I'm a total novice and learning as I go, but I'm putting together a RBM33G and a Sierra EM7455 right now. (I've also ordered an EP06-A, but it's on a slow boat from China and won't be here for a few more weeks.)
Here are my thoughts so far:
I updated the modem firmware to generic (I'll mainly be using this for Sprint)
I immediately flashed RouterOS v7beta8 to the RBM33G hoping to have the best compatibility with the modem. I had the LTE interface, but no band selection.
I tested AT&T and Sprint sims, and had internet connection but couldn't issue AT commands to the modem. I also kept losing connection on Sprint, but I've seen others complain that happens with the EM7455.
I flashed to GoldenOrb hoping to gain more control over the modem.
Flashing GoldenOrb was really easy with my windows desktop. Just be sure to backup the RouterOS license key first, otherwise you won't be able to revert back to ROS. If you've used RouterOS, you can probably handle the flash process. I used Tiny PXE and bootp rather than a serial cable.
I haven't tested WWAN internet connection through GoldenOrb, but hope to do that soon. I can, however, issue AT commands to the modem now - a huge win for me

I can switch bands, monitor connection strength, etc. Couldn't do that with ROSv7 and the EM7455.
I've never done any of this before, but I can try to answer questions about what I've done so far.
I picked up my 33g from neobits for a great price and quick shipping. They also have the 11g. I decided on the 33g because it was only a couple bucks more and my enclosure is going outside so size wasn't a factor.