Diagnosis
The electronic speed control (ESC) circuit board has failed or the potentiometer (dial) is disconnected. This is a known failure point.
How to Fix It
The speed control is not repairable. The entire switch and electronics module must be replaced. This is part of the upper housing assembly. Contact Ryobi service for a repair or replace the tool.
If That Didn't Work
Before concluding it's the ESC, test with multiple batteries. A nearly dead battery can cause the router to run at minimum speed only, regardless of the dial setting.
Heads Up
Running a router with a failed ESC at a constant, potentially incorrect speed is risky for certain bits and materials. You lose control over the tool's performance.