@Patric 

Do you mean the Archer MR200 stops working once you connected the RE210 to its ethernet port? 

And it should be the WPS button, solid red means that The Range Extender has established a secure connection, but the signal strength is too weak, while solid green indicates signal strength is adequate. Considering it flashes between red and green, the connection is unstable. 

Try to change another sockets to plug it, and ensure that the firmware is up to date. 

Besides, you can change the wireless settings on the main router to optimize the wireless performance. 

Nice day. 

@Patric 

When RE210 works in range extender mode, it will connect to the main router by wireless, and you cannot connect it to the router by wired cable, otherwise, it may stop working. 

In access point, we have to connect the RE210 to the MR200 by ethernet cable to get internet; so first please verify what the operation mode is. 

To optimize the wireless performance, first you need login to the web UI of the MR200.

And then go to wireless settings page, there you can change the wireless channel and channel width; 2.4GHz: channel 1, 6 and 11, channel width 40MHz; 5GHz: channel 36, 40 and 44, channel width 80MHz.

Besides, please ensure that the firmware of two devices are up to date.