I have seen a message from fourier on #zaurus asking if there's any 3G USB modem that works under pdaXrom, he went offline before I could answer him so I'll answer him here. I have never used one, but I'm curious to know what options for a 3G/LTE modem are available, too, so I have done some research.

From what I read USB modems usually work, but most likely all of them would require a powered USB HUB in order to work, see here: https://www.oesf.org/forum/index.php?showtopic=23465

IMHO the easiest solution would be to connect via tethering to a phone, using a wifi or a bluetooth card, wifi might be faster: https://www.oesf.org/forum/index.php?showtopic=21615

Softbank offered in the past a 3G CF modem, which was officially supported on the Zaurus, but you'll need a lot of luck to find one nowadays:

And I have come across a 3G SD modem made by Huawei. It's an SDIO card; such cards are supported by the later Z models, though driver support will be most probably an issue, only the latest kernels might support them (hence, only Alarmz):