If the device is wired with an Ethernet cable, I’m not sure what difference a strong WiFi signal would make, as it would be connecting via the cable, and not WiFi. 

Could be a simple network IP address issue, if you’ve already checked the cable and the ports. Try a simple network refresh by unplugging all Sonos devices from power. While they are all unplugged, reboot your router. Once the router comes back up, plug the Sonos devices back in, and wait a couple of minutes before you test. 

Thankyou for your reply.

I have this morning followed your advice and I was able to change songs etc for about two minutes before being met with the same message of “unable to connect to device, please try later”.

I can change songs etc for both room whilst in the room with the port in, but as soon as I go into the room with the amp I am unable to do anything. I know there is nothing wrong with the Ethernet cable as it is used with my pc etc so appears that there is a problem with the amp. 

Then I would recommend that you submit a system diagnostic within 10 minutes of experiencing this delay in issuing a command, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis.

Is the room with the Amp served by a different WiFi access point? An extender? Is there a WiFi mesh system?

The room with the amp has an Ethernet cable running to the same router that the port is connected to by Ethernet.

I have switched cable but still get the same outcome where I am unable to connect in the room where the amp is. 
I have run diagnostics from that room and the report number is 140957968

Thankyou all for your help.

after spending a substantial amount of time this morning with Sonos help on the phone the problem was found to be my sky q mini box. On switching the WiFi “off” on the box everything worked just fine. Just as well I have reasonable WiFi in the room without it!!!