3G/4G usb key and OpenWrt routers questions

Hello to everyone,

It's the first time I'm reading and trying the Openwrt os and I like the idea to have an updated configuration without buying new hardware with just few only factory official updates.
I already had a WL-330N3G wifi mobile router and I updated the firmware with the latest version and seems running great with such modern kernel compared to the official one.
Considering I would like to use a 4G/3G usb key, my question is if these modern or older usb internet modem have their own linux based firmware (often not updated)?
Cause I was asking myself if it'd be useless to have a latest 4.x kernel based router when using a 2.x or 3.x kernel based usb key that are often not configurable but still seems to have their own local area network, wouldn't it?

Much like cable modems, the firmware in cellular modems is always closed-source and very seldom updated. Usually the phone company has to agree to push it to the modem.

A good plan is to protect what you can, as best as you can. Yes, your USB "key" may, at some time, be compromised, but a good firewall between your valuable network devices and it is likely the best you can accomplish without purchasing discrete hardware.

Thanks for the answers. From a logic point of view I was thinking that the last network device before the wan should be the most updated one and some of these 3g usb modem seems to use a lan network themself for the usb connection but often with basically no configurations available for the localhost http connection.

About the 3g or 4g usb devices, is the Dlink DWR-730 usb mobile router (MT6280) supported if connected to a Openwrt usb router devices?