Hi Force Legato,
I'm afraid that I'm a bit at a loss in your case. As your WiFi-connected PC can cast to the wired M1000 it doesn't seem to be a router issue. Some routers don't allow multimedia packets to travel from wired clients to WiFi clients and vice versa but that's obviously not the case with your router. Unfortunately I couldn't find a user guide for that Plusnet device.
However, to rule out that something in your router's setup affects casting to your M1000 you might want to check if there are options named WMM, IGMP and/or Multicast or UPnP. WMM has to be disabled and IGMP/Multicast must be enabled. It might also help to enable UPnP.
Please also make sure that there's no security software (antivirus, firewall, etc.) active on your phone as that might also cause problems.
SoundBridge M1001 and M1000 • Clint L1 (retired) • DNT IP-dio (retired)
Google Chromecast • Amazon Fire TV • Amazon Echo and Echo Dot