Here is how things are hooked up. Cable from wall to splitter. Cable from splitter to Spectrum modem. cable from splitter to internet modem. ASUS SB6141, Cable from ASUS modem to router. DVR for cameras cabled to router. This is the way instructions said to do it.
There is no need to have 2 modems. When I had Spectrum they were also charging xx a month to 'rent' their modem. I bought my own and took theirs back, you just need to tell them that you bought your own and they have a listing on their website of what modems they will support. You could also then get rid of a splitter.
The hookup should be: Cable from wall to modem, Ethernet cable from modem to router, then plug all your other stuff into the router. If you have more things to plug into the router than there are connections then consider getting a switch