That remark triggered something. Ziggo has been moving to IPV6 with DS-Lite which uses carrier grade NAT for IPv4. (Dutch article - https://community.ziggo.nl/internetverbinding-102/ipv6-ds-lite-fupc-gebied-en-bridge-mode-waarom-zou-dat-niet-kunnen-30135)
That would probably be hard as the Ziggo hardware is closed. But RouterOS provides a basic packet sniffer in the tools menu (IIRC) so @Thunderwolf007 hould be able to check if any port 1700 packets arrive on the gateway itself.