You can try using the older RumbleTV channel/app https://channelstore.roku.com/details/458ecef80b412327152af7d77b343632/rumble-tv.
It is deprecated and can be removed at any time by Rumble. I like the old layout better but does seem to crash quite a bit. There were user complaints that the old version only listed 50 subscriptions. The new version of Rumble has solved all the crashing issues so definitely more stable than the older version of Rumble and RumbleTV.
If you plan on keeping more Rumble subscriptions than what the Rumble channel/app allows, you may want to look into some sort of casting ability from a laptop or mobile device through the Roku device. I generally keep between 20-30 subscriptions on Rumble so have not run into the issues experienced with users with many more than that.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941