If you are sure the black cable is coming from the Omnibox on the outside wall, then use it. If it does not work, unscrew the splitter and use the single cable end to try. You will not damage anything trying.
If it all fails, you will need a manned install as it could all be disconnected at the street cabinet.
It's interesting that not one of those cable ends are terminated with a cap, as if they are live, that's a sure way to introduce electrical noise into the whole street circuit ....
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