1. I'm using a router for better connectivity to the Ui 24R, can I still use your Ui MIDI app?

Yes. In the app, you can set any IP address of your Ui mixer as shown in the picture:

Ui MIDI app with router IP address

2. Error message "Unable to open port" when starting Ui MIDI app - Windows version

Solution 1:
FIRST connect you USB MIDI controller to tablet (PC) and THEN RUN the Ui MIDI app !

Solution 2:
If problem persist please delete whole folder /Uimixer under your Documents on C disk and set-up correct MIDI In and MIDI Out PORTS of your connected MIDI controller ! And always connect your MIDI controller before launching the Ui application!

It does not appear in the iOS version.