Need ext antenna capable modem for US/MEX

I am looking for an inexpensive, versatile and high performing cell modem for traveling in Mexico and the US that has a usuable antenna jack and will work on the majority of carriers in both countries. That would be all GSM with the caveat that Verizon LTE additionally would be a nice option, but not to the point of severely increasing cost of the modem, In other words I can more easily switch to another modem when needed, of which one could only support Verizon if necessary. A unique Verizon modem that had fall back to 3G CDMA would likely make more sense anyway.

The modem hardware can be USB / Express / PCMCIA and should work with a moderately priced Cradlepoint.

My reason for not just randomly purchasing one is I have read concerns which are beyond my current understanding of the architecture regarding the style of GSM that T-Mobile uses and that Telcel (Mexico) uses or is about to use with their 4G roll out that it's possible run of the mill GSM style modems don't support. I assume (from lack of hearing anything) that the other GSM carriers must not have these issues (US-T-Mobile, MX-MoviStar/iUSAcel)

Now with the external antenna jack the highest priority, followed by lowest price I am looking for the best and most flexible modem (USB/express/PCMCIA) I would not have a problem buying seperate modems if the cost of a single modem that supports all these carriers would he higher than buying 2-3 that cumulatively handled them all. Thank you!