There are two computers? One works OK but another one doesn't? Another one works with a different modem router? Did you try different ethernet cables to connect that computer and W9980? Since Wi-Fi...

Can you try the SIM card on your phone and test if it works in Spain for free? What's the connection status of web interface of M7350? http://www.tp-link.com/en/faq-794.html

Is it possible for you to connect the music system to your home hub directly and test if the network can see its components? You may have to change IP settings on the music system with the new router.

Download the MAC utility from the link as below. It works for PA8010P as well. No worries. :) http://www.tp-link.com/en/download/TL-PA6010_V1.html#Utility

You'd better try different web browers and different computers. If there is anti-virus software running on your PC, disabled it as well.

DHCP server is disabled on W9980, isn't it? Then it is not working as a router actually. Anyway, I should say, you'd better use W9980 as a router with DHCP enabled and Disable "Allow Guests to access...