Port mirroring is typically used for troubleshooting, or special traffic conditions (eg voice recorders for VOIP). It would not be a good idea to use this for the purposes you're describing. Instead, as suggested above you should be able to achieve your outcome by simply tagging the WISP uplink traffic onto an unused vlan to send it across the fiber to the MX WAN port - Both the WISP cat5 port and the fiber port going to the MX tagged as access ports. With that said, the best option is to have the ISP run the to MDF instead.
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