Welcome to Vodafone Community
I'd like to share the following settings with whoever needs them. I recently bought a Unify Dream Machine as an addition to my Vodafone modem/router. I quickly discovered that there's no way to set up passthrough or bridge mode on the device. I tried DMZ for a while, but you will end up with a double NAT situation, makes it hard if you want to use a single URL for Home Assistant for instance. So I kept looking and ended up buying a modded BT Home Hub 5 flashed with OpenWRT (cheap 20 pounds on eBay, don't pay anymore than that!).
I took the following steps to put the modem in bridge mode to work with my UDM. This should likely work fine for other routers too as long as you can put in your PPPoE login.
1. Get your PPPoE login details via the Help Chat. Pretty straight forward.
2. Follow this link to a very extensive guide to a community made manual for the BT Home Hub 5 running OpenWRT: https://www.scribd.com/document/530053813/1-OpenWrt-LEDE-Installation-Guide-for-HH5A
3. Scroll to page 112 (section 9.8) and follow the steps. Some notes on the steps you have to follow according to the guide linked above:
- The settings I used for VDSL2 are the exact same as the second screenshot on page 113: Annex B (All), A43C + J43 + A43, PTM/EFM, VDSL.
- I went for the alternative option and used LAN1 as the WAN port to connect my own Dream Machine to. (the third image on page 115)
- On page 118 it's important to follow the Openreach VDSL2 steps and enter "dsl01.101".
- I disabled DHCP on the BT Home Hub too.
4. Connect a cable from LAN1 on the BT Home Hub to the WAN port of your router. Navigate to the WAN settings of your router and change these to PPPoE and fill in your login details you received from Vodafone.
5. Restart and it should work, you now have put the BT Home Hub 5 in bridge mode and no more double NAT solution.
I'm confident to say I'm pretty good with tech and configuring stuff, I'm in no way a network expert and I'm happy I got this to work, however if you have more experience and you see anything I could do better in the guide above, please do let me know so I can edit this in the guide and test it myself and improve. The purpose is to easily help people who are in the same situation as me and want a little more control over their home network.