As a quick guess, your Era speakers are in a separate Sonos ‘room’ and grouped, rather than set up as the same room as surrounds. 

Sonos only allows an TV remote to control the one Sonos ‘room’ that the soundbar is in, and not any other ‘rooms’.

Note the quotes around ‘rooms’ in my post. I’m talking about the concept in the Sonos software, and not in the physical sense. 

Are the rear speakers added to the Ray as surrounds instead of a separate room?  Because they need to be configured as surrounds to be controlled by the TV remote.  If they are grouped, they are not the same room (among other problems, like a delay echo when viewing TV sources).

Initially i did have them “grouped” but i later went back to the app and set the ERA speakers as Surrounds in the app. As for them being in the same “room”, the app shows all 3 speakers together in the the same room when they’re playing. I’ve been on hold with SONOS support for about 45 mins and they just answered. The rep confirmed to me that the only way i can control the rear speakers is through the app? Surely this can’t be correct???

Despite the fact i later set them up as surrounds, maybe since they were initially set up as a group, they still are? Is there a way to “ungroup” them to check to see if that’s the problem?

Go to Settings > <Ray room name>, under “Products” you should have the Ray and two Era 100’s.  If you don’t, they aren’t set up as surrounds.  Go back to the Settings menu.  Choose the room with one (or both) Era 100’s.  If they are paired in the same room, you need to unpair:

https://support.sonos.com/en-us/article/separate-a-stereo-pair

Then add as surrounds:

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

Ok. A little progress. I now have both ERA’s confirmed successfully set up as surrounds. The only issue now is that there’s no sound coming from either one? Only my sound bar.

Update - once I went through Trueplay setup, the speakers worked. Didn’t realize that was a requirement.