hello DannyD
Welcome to the community.
Adding an access point to your network will sure help with the wifi coverage. Using the ethernet cables is the way to go. Fast wifi runs through ethernet.
Adding a third party access point might result in short connection losses when switching between access points (Connect Box 3 / Ubiquity). You could turn of the wifi on the Connect Box 3 and get another Ubiquity access point or you can rent WiFi Extenders from Sunrise to extend the wifi.