<<< the network icons in the taskbar have yellow triangles with exclamation marks – status is ‘limited or no connectivity… the network did not assign a network address’. >>>
This suggests that the adapters are set to get their IP addresses automatically, but have been unable to find a DHCP server from which to get them. Windows will almost certainly have allocated a default address in the 169.254.x.x range which will result in the Limited or No Connectivity error.
If it is your intention to have "shared" internet access with this arrangement, you need to enable "Internet Connection Sharing" on the actual Internet connection (the "modem" connection) in the Network Connections folder. Right click the entry, select Properties, then the Advanced tab and tick the box "Allow other network users...etc".
This will automatically configure your Local Area Connection (the Homeplug USB adapter) with the fixed IP address 192.168.0.1 and subnet mask 255.255.255.0
NOTE: If the PC already has an existing Local Area Connection (i.e there was already a network adapter installed), this should be disabled by right clicking and selecting Disable, before setting up Internet Connection Sharing. Otherwise, the 192.168.0.1 address could be allocated to the wrong connection.
The "Client" PC's Local Area Connection should remain set to get its IP address automatically, which it will now do by DHCP from the Host PC.
If you are NOT sharing an Internet connection and ICS isn't enabled on the Internet Connection, you need to allocate the IP addreeses for the two adapters manually. You can use 192.168.0.1 and 192.168.0.2 with the SAME subnet mask address in both of 255.255.255.0