I'm going DHCP and my Airport Extreme BaseStation does not show me what IP Address it assigned to it. However, I know how my browser reacts to an authentic IP address versus one that is not. Myminer.io gave me the IP as it did all 14 other rigs. I could connect to all of them just fine except for this one.
I have my Airport set up for DHCP and NAT
Wonder if I should do DHCP only?
Sorry I can't really comment, I'm still learning networking as a whole (and mining has been invaluable to make me have to learn). In my situation, I wanted to narrow down as many variables as possible, so I directly connected the SP20 to my PC via ethernet, disabled wifi, and ran tptpd32 to create a dhcp network for the SP20 outside my LAN. Once it has assigned a lease, it lists the devices in the software. I was then able to get to the GUI via web browser and set it up as normal, and avoid whatever IP it was being assigned from my router that would not allow me to connect to it (even though it would show up in the myminer.io interface).
I had success using DHCP only, so if I was you I would try it. And use a program that lists the leases so you can be sure it is connecting. You did try different ethernet cables right?