MTU connection setting suddenly needed changing

My setup:
- Virgin Media router acting purely as a modem (WiFi disabled).
- Flint router connected to the Virgin Media modem via Ethernet, providing WiFi and Ethernet to all my devices.
- ProtonVPN connection configured directly on the Flint router (WireGuard).

Issue:
This morning, my internet suddenly stopped working on all devices connected through the Flint router. The connection from the Flint router to the Virgin Media modem was still up (I could see an IP address assigned), but traffic wouldn’t pass through — the VPN tunnel failed to establish, and even direct pings didn’t work.

Cause / Fix:
After some investigation, I discovered that manually adding an MTU = 1280 line in the WireGuard configuration file fixed the issue. It seems that Virgin Media made a network change on their side (likely related to packet size or NAT handling) that broke the previous MTU setup, which had been slightly higher.

Notes / Lessons learned:
- When using WireGuard over a router behind Virgin Media, small changes in MTU can completely break connectivity.
- If your VPN suddenly stops working but the physical connection is fine, check MTU settings before looking for more complex causes.
- For Flint/OpenWrt users: MTU changes must be reflected in the LuCI interface or the .conf file for WireGuard; otherwise, the VPN may silently fail.