The 3G router/gateway coverage is not the problem but the remote serial link connection and the comm port setup on the connecting PC I have to do some thinking about.
Don't really understand.., thought I'd answered that but maybe not.
A device server - something like a UDS1100, but there are many alternatives - has a high-speed serial input (think I've seen it quoted up to 920k baud, but certainly 230k baud) and an Ethernet output. If you connect the UDS1100 output to a 3G router (but a router and not just a modem) then you can (potentially) pump data in at 115k so that's not the bottleneck. The limiting factor is the bandwidth that the 3G link has available at any one time. What always tends to be quoted is the _maximum_ bandwidth, which could be eg 3.6 Mbps, but typically what's available is _much_ less. If (more likely, when) this dips below 115k then you're in trouble.
But if you can get the speed and reliably then standard Com Port Redirector software on the target PC should reconvert the IP stream into a serial stream.
There's a separate discussion about the cost and specification of the hardware - MBB router/modems tend to be more costly, though there is the alternative of using a standard office router of the type that accepts a 3G dongle for backup connectivity (I know Draytek and Billion make some and I'm sure other makers do too). I've never tried this sort of set-up with an Envoy8x or with high-speed serial but can't see any reason in principle why it shouldn't work, provided you can guarantee sufficient bandwidth on the 3G data link, which is a big if.