I have a Spectrum cable modem, TP-Link router, and two Ubiquiti PoE access points (each with a PoE injector). I'm looking to replace the TP-Link router and PoE injectors with a Gigabit hEX and hEX PoE; will this work? Seems like it should, but I wanted to check before buying.

Be careful about MT poe, on some of the lower models it is proprietary MT poe and not to standard that other equipment uses!!
It looks like the hex poe does on the output side follows --> PoE out 802.3af/at So good to go there......

So it depends on the total draw of all poe devices...... for example two ubiquti U6LR draws max 18.5 watts and two of them would be 37 Watts whereas the standard for at is 30W, so the hex would not be enough for example. However if it is soemthing like U6 pro or less 13W, should be fine!