Is it just me, or is the port forwarding really limited on the 2820n?

It seems that you can only forward a single port per entry, and there are only 20 entries. If I try to forward a range of ports, such as 2300-2400 then it tries to forward to 100 different IP addresses too... it's REALLY annoying. What's also annoying is that you can't setup a port forwarding rule for both UDP and TCP.. you have to create one rule for each.

Has anybody found a way to create a forwarding rule for both protocols and also for a range of ports to a single internal IP address?

You probably need to look at 'Open Ports' (also under NAT) which allows you to do (most of) that. Up to 20 hosts with 10 port ranges each. Only restriction is that the internal port numbers have to be the same as the external ones.