With the Google Mesh systems (Nest), you can't do the above. The points are really just "extenders". (no seperate configurations). The router also can't be configured for address reservations or even selecting wireless channels. (These are really just plug and play and just about everything is automatically managed by the firmware).
You really are left with just playing around with the locations of the router, the points/extenders, and the Roku devices themselves. Maybe move the point that keeps connecting closer to the Roku device, and vice versa with the point that doesn't like to connect. Try moving them a few feet further away from the device. (can't discount that interference from being too close is causing adverse behavior). Close proximity of equipment sometimes actually creates connection and stability issues due to amplified interference. (depends on equipment being used and other factors in environment).
Some of the problem is with all this automation/automatic management. The router is continually scanning and changing channels to avoid crowding/interference, so what may be a good connection one moment, may change to a bad connection the next. This likely leads to an unpredictable 50/50 chance of which point a device will prefer to connect to. Might even have to resort to unplugging a point, let the Roku connect to the only one broadcasting, and then plug the other one and hope the Roku device does not decide to switch to the other one.
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