Firewalla Router Mode behind ISP Bridge – TCP reset

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Fabrizio Acquati

• January 18, 2026 16:34

ISP router (bridge)
→ Firewalla Purple (Router Mode)
→ single wired PC

Firewalla WAN gets public IP.
DNS works.
Ping partially works.
HTTPS/TCP fails with ERR_CONNECTION_RESET / ERR_CONNECTION_CLOSED.

Issue occurs even with:

• one PC directly on Firewalla LAN

• no Wi-Fi, no mesh, no switches

Tried:

• MTU 1500 / 1492 / 1460

• IPv6 OFF

Matches this known issue:
https://help.firewalla.com/hc/en-us/community/posts/6683295637907

Other routers (AmpliFi) work in the same bridge setup.

Question:
Is PMTUD / MSS handling broken for forwarded traffic in this scenario?
Any workaround?

Thank you for the help!

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  Firewalla

  • January 18, 2026 18:25

  "Ping partially works." this is likely the main issue. You have packet loss on the network and that's causing issues. Have you tried changing cable on the WAN and LAN side? if you have already, please let me know, I can create a case and look inside the purple

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  Fabrizio Acquati

  • January 18, 2026 19:34
  • Edited

  I’ve now performed the fallback by removing bridge mode on the ISP router, and Firewalla is configured in Simple Mode (which is the setup I don't want to keep using).

  The ISP router is now acting as router and DHCP server, while UniFi is providing Wi-Fi (mesh).
  Firewalla’s LAN is connected to the UniFi mesh.

  The cables I’m using now are the same ones I used when Firewalla was acting as the router.
  I’ll buy a new set of cables anyway.

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  Firewalla

  • January 19, 2026 00:07

  You were using router mode? and it was not working? now you switch to simple mode? 

  Or your Firewalla always in simple mode? Simple mode is not perfect, it does cause issues if your main router doesn't support it. 

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  FirewallaSupportDesk

  • January 19, 2026 00:50

  If new cables don't make any difference, please reach out help@firewalla.com with the link of the post, so our support can quickly jump in and help you out.

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