Why won't my Asus ZenPad Z170C use a 255.0.0.0 (network prefix length 8) network?

[AC Question]

Hi

I'm trying to connect my ZenPad to a network which requires a 255.0.0.0 subnet mask but it won't seem to change from anything other than 255.255.255.0!

Android v5.0.2

Many thanks!