@bigbarthy,

For a gigabit connection, you can use either port. If you have no need for the multi-gig connection for your local network, I would recommend first trying the 2.5 Gbps port.

Your cloud device should be in reference to the router that you purchased and set up. The connection to the TP-Link Cloud is what allows you to use the Tether App to see your network while away from home. 

What you are experiencing is pretty typical for a wireless connection, as there is a falloff in bandwidth from interference and simply due to the extra data that must be sent with wireless connections.

Normally that ~600 max is a result of the wireless band that your device is using to communicate, as those speeds normally mean you are connecting to the 2.4GHz network. You may try separating your bands by turning off smart connect and connecting your devices to the 5GHz network that would appear.