Between the WAN port and the ISP handoff you'll need a VDSL modem (for example Draytek 130) which converts PPPoE to PPPoA. The MX will then establish its connection as PPPoE client and the static IP will be automatically be picked up - you don't need to (and can't) set that address on the interface.
Every time I've deployed in this way, the only configuration required in the MX is PPPoE username and password. The modem should be in bridge mode - not PPPoE mode - the MX then becomes the PPPoE client on the network and the modem (in bridge mode) is just a passive device that converts PPPoE to PPPoA. If you use the Draytek 130 modem, you just un-box it and plug it in. No config at all required.