what brand/type of network switch are you using as a hub for the xdjs/rekordbox? is the hub connected to any other device/router/switch/etc besides your PC and two xdj-1000mk2? sounds like you've covered all your bases so my first guess is that :
idea #1: your switch is mucking things up. super simple unmanaged hubs seem to work the best, particularly when used as dedicated rekordbox/dj hubs (ie. not connected to your larger home network and/or the internet).
idea #2: try turning off your PC/Mac wifi, allowing rekordbox to LINK with Pro Link network solely via a wired (ethernet) connection. "
idea #2.5: just for fun, if you have the cat5e LAN cables that came with the xdjs, try those and/or any cat5 cables.
idea #3: if you were using a DJM mixer with Pro Link capabilities, i'd suggest making sure the xdj player number matches the mixer's channel number. i don't think it matters as much with a non-pro link mixer, but try setting each player number manually; and make sure that both players have 'on air' turned on.
idea #4: download and install the PRO DJ LINK BRIDGE - the xdj-1000mk2 isn't listed officially as a compatible 'BRIDGE' device, but they show up on the Link Bridge utility none-the-less. if there's a Pro Link network issue happening, the utility will usually see it: https://www.pioneerdj.com/en/product/software/pro-dj-link-bridge/software/overview/
let us know how things turn out. m+