My solution to budget 4G LTE failover on Sprint

Raffi_

After a lot of pain, I finally got my setup working so I thought I'd share to hopefully save others the same pain.

Background:
We needed a 4G LTE failover in our small office but budget was tight. We tried to go with a Netgear LB1120 with a Google Fi data only sim since we already had an active Fi account and we would only be billed for data we used. Perfect! Except that T-Mobile data rates in our building are horrid. Took me a while to figure out that even though Google Fi operates on T-Mobile, Sprint and US Cellular, their data only sim only operates on T-Mobile, bummer.

Side note, I think the LB1120 should be a great solution if you have good service from just about any network other than Sprint.

After lots of testing, of course only Sprint provides decent enough bandwidth in our building. This is a step that everyone should do. It's called a "4G LTE site survey" in case you're looking up more info on how to do it. Basically test a 4G device on every network in your area and usage case to see which has sufficient signal, speed, latency for your need. I skipped this step because I figured any 4G network should be decent enough as a backup. Wrong, and I ended up with the first device (LB1120) which was now useless for us.

...On to the current solution.

Hardware on a budget,
Used Cradlepoint CBA750B off ebay for super cheap
Used Netgear 341U USB modem off ebay for super cheap

Network on a budget,
Tello MVNO operates on Sprint. They offer a $5/month 500 MB plan. That's not a lot of data, but should be enough for an hour or two with our normal traffic. The nice thing is, if our main gateway goes down I get notified and I can go into the Tello account and increase the data immediately and pay for that increase as needed rather than paying $20+/month for data we never use.

Put the Tello sim into the 341U.
Plug the 341U into my Windows 10 laptop. Logged in with default creds and then performed a factory reset. This was the easiest way I found to get the modem to reactivate the new sim, download PRL updates and all that good stuff.
After that was all done, no configuration on the 341U modem was needed other than changing the default password (I think it's a good idea to do that).
With the same laptop, connected to the ethernet port of the CBA750B. Logged in with the default creds and updated the CBA750B to the latest firmware. The firmware was a little difficult to find on the cradlepoint site. You can find it here, https://customer.cradlepoint.com/s/NetCloud. Just go to the Select Device drop down under NetCloud OS Downloads.
Kept the CBA750B in ip-passthrough mode (the physical switch on the box in the "I" position).
Plugged the 341U into USB port 1. When the CBA750B is in ip-passthrough mode, it will also automatically configure the 341U into ip-passthrough mode. So now the 341U and CBA750B are both configured for ip-passthrough. Meaning the IP received from Sprint is the IP passed to the ethernet port of the CBA750B. Perfect, I now have a gateway!
Plugged the ethernet port from the CBA750B into an unused interface on pfSense.
Logged into pfSense and configured that interface with the settings below. Notice the alias IPv4 address? That's so I can talk to the web GUI on the CBA750B. Oh, that's right I forgot to mention I changed the CBA750B default password and default IP address to 192.186.20.1. This is of course optional, but thought I'd mention it since the alias is needed to talk to it.

Created the Gateway group below.

Set the default gateway to the new gateway group.

.. and that's pretty much it.

Now you may be thinking, this seems super simple (and it is), so what is all the pain this guys is talking about? Part of the pain was skipping that step of not doing the site survey. The even bigger pain, was the fact the 341U modem was constantly disconnecting and reconnecting to the Sprint network in the original location I had the gateway placed. I ended up moving the gateway into my lab to make it easier for me to troubleshoot and of course when I did that, the problem went away and it was working perfectly stable. I can only assume this was because the signal was not strong enough. The signal bars on the front of the CBA750B are misleading because I believe they represent the 3G connection and not the 4G connection. I believe the single bar I had on the LCD of the 341U is more accurate but even in the lab location where it is stable and working fine, it's still one bar (go figure). I'm pretty sure the RSRP numbers didn't change much either. I'm no expert on RF signals and I couldn't figure it out. All I know is it works in my lab, so that's where it now lives. The CBA750B supports POE so I power it with that and mounted it on the wall in my lab, with an ethernet cable already available there going to the front of our office's patch panel where pfSense lives. That's good enough for me.

Caveat:
If you have a tight budget or can't convince superiors (boss, spouse) to spend more on this project, I would sort of recommend this setup, but I don't highly recommended it. If you run into problems like I did, it isn't the easiest thing to troubleshoot. I'll try to help you out, but don't look at me if it isn't working. It took me a LONG time to figure out. If you don't have time to spend on figuring things out like that, spend the extra money and get a proper 4G LTE gateway.

Good luck,
Raffi