OK thank you for your suggestion BluishHumility.
I have uninstalled Tailscale and setup the Static IP all fine.
My ISP is indeed Virgin Media based in the UK… and the reason I’m trying to fiddle with static IP inside the software instead of the Router is because the router doesn’t allow me the change the DNS inside it and I want to use PiHole that is installed within the dietpi image as DHCP server so that I bypass the DNS config… chicken and egg hey…
Will try to reinstall Tailscale and report if any issue once installed.
Thanks a lot for your help all!