First off, confirm what the 2 networks are by checking the IP address of devices attached to each.
On a PC, what you can do is open a CMD window (dos prompt), then run IPCONFIG
This does not change anything, justs lists out a load of details. You need to find in there what your IP address is, probably titled "IPc4 Address" and something like 192.168.xxx.xxx
on a network, you would expect that the first 3 numbers (everything up to the final decimal point) would be the same. IF you have 2 networks, that will not be the case.
(sorry if I'm teaching granny to suck eggs).
I'm not familiar with PowerLink or PlusNet, but it could be that it gives 2 IP ranges, one for the wifi connections, and one for the cabled connections. If so, and you are right - it should be able to route between them - there may be a setting in the configuration screen to set the IP ranges, and you could set them both the same.
I had a similar problem, and it ended up being too many cables between the cabled network and a wifi booster - I had connected them together with 2 cables (in parallel if you like), and the poor wee booster took it upon itself to support 2 network ip addresses.
Merv.