VPN L2TP only from mobile network make issues

I’m uncertain whether this issue started after the iOS update (possibly) or the RouterOS update (currently 7.17). Everything worked fine a few weeks ago before upgrading iOS and RouterOS.

This is a very strange issue, and I’m not sure how to debug it effectively.

Here’s what I’ve observed:
• MacOS → VPN: Everything works.
• MacOS → USB Phone Hotspot/Tethering → VPN: Everything works.
• iOS on WiFi → VPN: Everything works.
• iOS on Mobile Data (4G/5G) → VPN: It connects, but some websites (e.g., google.com, Cloudflare-protected websites) don’t work. Others, like nba.com, work without issues.

Additional details:
• Enabling or disabling Private Relay makes no difference.
• Tested with multiple mobile carriers across Europe and several iPhones, all running iOS 18.2.1.

I suspect this might be related to HSTS or a similar mechanism, but I’m unsure.

Can anyone confirm or deny this? Any suggestions on where to look or what to test would be greatly appreciated.

Thanks !

Found a solution here: http://forum.mikrotik.com/t/iphone-l2tp-issues/175502/1