If you change your DNS settings to activate wao.io, you may come across existing AAAA records for IPv6 addresses. wao.io does not accept incoming connections via IPv6. As some clients prefer IPv6 records over IPv4, you should remove those AAAA records in order to have everyone using wao.io.
Some hosters don't allow you to remove the AAAA records from the DNS. They might use their own IPv6 services as default. To workaround this behaviour you can use the IPv6/IPv4 compatibility notation to address our IPv4 systems:
IPv4 address IPv6 address
Please refer to the DNS configuration docs to choose the right IP addresses.