21.0 won't work with a 612. Been through that before. They ship now with a 20.0. Otherwise, for uhf, you'll need a slightly older model such as a 6.4 (which they used to ship with).
Do you still have the remote you're replacing? If so, if it works even halfway, try to change the address with the OLD remote to an even number like two. This will make it band b vs band a if uhf. Now with the new remote make sure it's got the '1' tab, uhf on the rear switch.
Sometimes this trick will make a newer remote compatible with an older receiver.
Before you ask, 'why then did they send me this one' because that's the new remote they ship out. Specifically request the model you need or ebay/amazon it.