Same problem, same remote, same receiver. I also have the remote that came with the 508 receiver. Both worked fine with the 508 (IR and UHF) but neither will work in UHF mode with the replacement 512 receiver I just got. Nor will the 5.4 remote that came with the replacement but it may not be UHF. Called Dish and they are going to send me a new remote but of course that will not help as the 2 old IR/UHF remotes worked with the 508 as late as yesterday and are fine. I think Dish just sent out another defective reconditioned receiver (seems to be their thing) as a replacement, but am open to suggestions. Again my settings are as recommended above - 1 shows in both sys. inf. menus, UHF is enabled and the antenna (from the 508) is attached to the back of the receiver (took it off and reattached it but no help of course). I even blocked the IR imput on the 512 and pointed a remote at the antenna from a foot away. Nothing. Thanks.
BTW, Hermit did it work for you. You never said.