The IPv4 address textfield in Wizard => NATTED mode (and also in network settings) does not pass the browser's HTML5 validation
_In natted mode, all qMp devices in the mesh network need a unique IPv4 address with a /32 netmask. If unsure about which one to select, leave the field blank and a random one will be assigned automatically
*HERE THE BOX*
Write the mesh-wide public IPv4 address for this device with a /32 netmask, or leave it blank to get a random one._
Leaving the box blank works OK (i.e., to get a random IPv4 address).