The HR21 definitely supports RF...and no external antenna needed. Is your remote an RC65R remote? (I believe the current ones being sent out are RC65R, RC65RX, and RC65RBX)
If it has an R after the model number and an FCC label on the back it's an RF remote. The RC65 and RC65X are not RF remotes.
If it has an R after the model number and an FCC label on the back it's an RF remote. The RC65 and RC65X are not RF remotes.