Try connecting Roku directly to the 2.2 input on the TV, go through basic setup, confirm 4k content is available, then power off and move to the Marantz.

I had to do the same w/ a similar setup + Crestron DM last spring. Since then the Roku has 'lost' the 4k content once or twice (presumably based on a failed 2.2 authentication) and required a power cycle. Haven't had to repeat the above setup process, so not sure if that was related to first-run, (followed by beta) DM firmware from Crestron instead. Either way bypassing the AVR is a good test.