Do you have another device to try the Roku with? Like an actual TV with an HDMI input? That might help narrow down if it's the Roku, the connecting cables and/or adapters, or the monitor.
Basil
https://www.basilsblog.com/
Roku Ultra (4660)
Apple TV (5th gen), TiVo
Previous:
Roku boxes from every generation.
Apple TV (2nd, 3rd, 4th gen)