NTP: Option to ignore DHCP provided server(s)

[UnderEu]

My ISP router (that I can't replace of reconfigure myself) provided a specific NTP server address that's not working as intended and causing my systems to be out-of-sync. I want to use something else for NTP sync my RPi running the latest RPi OS to date (Lite) but, despite editing /etc/systemd/timesyncd.conf to point the servers I want, it insists to get the ISP box's NTP address.

I found a plethera of posts and forums regarding the issue but every one is out-dated and/or didn't work at all.

And before someone asks: Setting my RPi to static addressing is not an option

Any help?
TIA