We currently use a 2860n router but are thinking of updating it to a 2860Ln so that we can use the 4G functionality if we loose the landline. My question is "can we use a 4G SIM from a mobile phone or does it have to be a special data SIM?"

I just use a normal sim. However there is no need to switch routers, just get a USB 3g/4g dongle and plug that in. I use an old Three one which I unlocked using a code from dc-unlocker.com and pop in a vodafone sim when required.