Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
If your Spectrum box offers DLNA streaming (some do, some don't), you can use the Roku Media Player channel to access the DLNA server. However, your recordings have to be in a compatible container and use supported audio and video codecs. I doubt the Spectrum box is able to transcode the videos if necessary. If your Spectrum box media can be accessed by a computer on your network, you could install a DLNA server on the computer, map it's media source to the Spectrum box, and use it to stream the media to your Roku.
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.