At home in my network (MX65W) my wireless settings are: MR42E (gateway) + MR30H in Mesh configuration. This functions very well. The only thing is my Samsung TV (somewhere in the middle): sometimes it connects tot the MR42E, other times tot the MR30H.
Signal quality is much better when connected with the MR42E.
Is there a way to "force" the TV to connect with the MR42E (without creating an apart WiFi network) ?
You could also try turning the power down a bit on the 30H to make it look less desirable to the TV. You'll need to be careful with this as you don't want to impact your mesh or your coverage area, but if they're all going full power now then you likely have some room to play with.