Add extra mesh extender? AC1900 Dual Band - EX6410

yes and no. 

Reason why is that the mesh extenders don't have something controlling them like a full mesh system like orbi. They connect to the routers ssid. And when they're running in mesh, they duplicate that ssid. When you have more than 1 extender, if they can "see" each others ssid, they can end up connected to each other and not back to the main router. 

It doesn't happen everytime and not everyone has issues. But if you do install a 2nd extender, if you run into the issue, simply set one of them up with their own ssid and it'll prevent it. 

The other issue you can run into is if you're daisy chaining them. (router------>extender------->extender)

Reason for that is the way extenders work is they receive and then retransmit a signal. And it can't do both at once. So it cuts the speed that the extender is receive in half. Add another hop to it and the 2nd extender drops the speed its getting in half. So by the 2nd hop, you're speed is significantly slowed. 

So if you're running more than one extender, you should be trying to go with a star configuration (extender<---------router-------->extender)

I usually recommend anyone needing more than 1 extender, to look to moving/upgrading to an actual mesh system like orbi. its designed to run with multiple nodes and it has a controller. Tends to have much better performance, stability, and ease of use.