So you have effectively 'spliced' all 12 connectors in the middle with that switch in order to extend them? With an unmanaged switch that would create a broadcast storm, with a managed switch my guess is it's detecting this and killing the ports in order to save the network.
If you can get away with it, I'd just use one line to extend the network. If you actually need all the bandwidth of all 12 lines, I'd suggest looking into something like fiber that can handle higher bandwidth and range in one cable.
Maybe the switch can be configured to 'segment' the lines into 12 pairs, but my knowledge of managed switches is limited.
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