Hi Thomas
You can read how to connect the IR extension board: [solved] How to connect IR receiver to the plate amp (2 channel) - #5 by Marcelch
For the web interface you need the IP address of your amp. You can find it in your 4STRAM app. Select your amp, click on the settings and then speaker information (i use it in german, so i hope the translation is ok). There you find the IP like 192.168.1.11. Then you can enter this address in your web browser on your computer. If you stuck on the loading screen, you have to factory reset your amp first.
To the qestion of scohrss:
My Yamaha subwoofer has no frequency settings, so i have configured the crossover in the ACPWorkbench in my plate amp. If you use your sub with a cable i think you also have to do it that way.