If you absolutely need to use this monitor, you need a HDMI to RF converter/modulator box.
The reviews of these from AoeSpy indicates works with Roku devices.
You can search for different HDMI to modulators as well, the above seems most reasonably priced and the reviews indicates users found it compatible with Roku for older TVs. It is recommended to use these with a power supply at the wall, so make sure to either have, or purchase a 5v power adapter, (5v, 1-2amp cell phone charging adapter should work), if the devices don't come with a power adapter.
Not sure if your monitor has speakers. If not, the first link seems better so you can add speakers to the setup if needed via RCA. If needed, you can also get RCA to 3.5mm adapter for computer-type speakers. (ie. https://www.amazon.com/VCE-2-Pack-Plated-Female-Adapter/dp/B06XCR898P/ref=sr_1_1?crid=3N99EAO8FH09R&...)
Basically, you will need to do a little retrofitting to get this monitor to work with streaming devices, but should work in the end. Of course, budget allowing, you could just add a TV to the room, or a smaller portable TV with an HDMI port that you can just mount to your exercise machine. https://www.amazon.com/s?k=portable+TV+hdmi&crid=1W90GW8LIBYWG&sprefix=portable+tv+hdmi%2Caps%2C712&...
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102