While using VPN, some websites open on phone but not PC

[freelancer]

Setup:

1. I'm running OpenWRT on my Belkin RT3200 router.
2. I'm running Wireguard server on a Vultr VPS.
3. I'm running Wireguard client on the OpenWRT router.

Issue:

1. Some websites (like duckduckgo, yahoo) show "connection timeout" on my PC browser. Others work fine (like youtube, openwrt.org). This behavior is consistent on two PCs (Windows: chrome and firefox). Same behavior on one of my phones (android: firefox, chrome).
2. These websites work fine on my devices when I don't use the VPN.
3. On another one of my android phones, these websites ARE accessible while using the VPN on the same wifi network. Very confused by thiis.

Troubleshooting:

1. I checked on all devices that my external IP matches the VPS IP. So I'm sure they're all using VPN.
2. I checked that all devices show no DNS leakage. They're all showing Cloudflare.

Would love to get some help on how to debug this and get those websites working on all devices.

[egc]

I have seen this kind of behaviour when MTU is too high.
So try lowering MTU on server and client.

Max MTU is 1420 and when using PPPoE 1412.
But sometimes you have to use a much lower MTU e.g. 1280

[wilsonyan]

make sure you have clamping turned on in the firewall section
if you only need clamping for the vpn you can create a zone just for that eg:

[freelancer]

Thank you! MSS clamping fixed it! I'm curious why it isn't turned on by default.