Well now, I've done a bit more digging and I have the option "Use Peer DNS" ticked in my router - I thought that this took a lower priority over a manually entered one but evidently not.
Long story short, I've been using CISP's DNS server all along and not the manual one I entered. \
I've unticked that option and now am actually using 8.8.8.8 and voila! I can now ping 23.5.107.206 successfully
Edit: but now other things broke grrrr
Edit2: By other things broke I'm referring to Netflix which started misbehaving, so I switched "Use Peer DNS" back on (so using CISP DNS be default again) and Netflix starting working properly but the
www.apple.com broke again...
Sigh