With iOS 11, Apple has radically reduced access to underlying Unix routines that many networking apps rely on. It is simply not possible to reliably get MAC address on iOS 11 currently. More info: https://netanalyzer.helpscoutdocs.com/article/119-mac-addresses-arent-available-under-ios-11
The only silver lining here is that the entire dev community is working with Apple to try to find a solution and as soon as one is found we will update IP Scanner to take advantage of it.