Locking Verizon Pantech UML-290 / Novatel USB551L Modems to 3G/4G Mode

It's possible to lock the Verizon Novatel Wireless USB551L modems or the Verizon Pantech UML-290 modems to 3G-only or LTE/4G-only modes.

This can help in the following situations:
1. the modem doesn't connect properly, or takes a long time (5-10 minutes) to connect; this is often seen with the USB551L
2. when there's mixed 3G or 4G coverage and the modem might have difficulty auto-negotiating or roaming between modes
3. when you're in a 3G-only area

These links mention that Verizon has confirmed the modems have difficulty switching, but we haven't seen a firmware update released to address this so these instructions can be used as workarounds:

UML-290:
http://www.proxicast.com/AbsoluteFM/afmviewfaq.aspx?faqid=224

USB551L:
http://www.proxicast.com/AbsoluteFM/afmviewfaq.aspx?faqid=225

With the fairly low cost of these modems (~$50 on eBay) it's useful for customers to have backup modems pre-locked to the 3G mode to swap out if they find they're in mixed service areas. This avoids having to swap the same modem repeatedly between modes when they're trying to get up and streaming as quickly as possible, and avoids the need for have a Windows computer on hand in the case of the UML-290 since the utility is Windows-only to make the switch.