I have a mesh system ax1800 and one of my routers has a red light on and not working. I pushed sync

What model do you have? AX1800 is a speed, not a model. 

You put "one of my routers". Most mesh systems have 1 router and satellites. 

Its the satellite having issues? 

did you try hardwiring it into the primary router and syncing it? 

What firmware version is on the router? 

and on the satellites? 

Are you connecting the satellites hardwired or using a wireless connection?