Try using the Network Reset in the Roku's settings menu. This has often resolved the issue for other users. Another option is to change the SSID on your new network to match the old network. Assuming the Roku devices can see the network they will automatically connect to the last used network. Of course, it's possible that be attempting to connect to the new one it will no longer do that. But it's something else to try if the network reset doesn't work.
Dan
Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.