@David-TP 

Hello,

Thank you for your reply regarding the beta firmware.

I understand that the beta version you provided automatically syncs the MTU from the SIM service provider.  

However, I found a post here on the TP-Link Community where user @davidtmx reported that he tested a beta firmware on the TL-MR6400 V4 and was able to manually change the MTU to 1430:

https://community.tp-link.com/en/home/forum/topic/229124

Quote from @davidtmx: "I've just tried the beta firmware and… It works!! MTU changes to 1430."

Could you please clarify which beta firmware version this was, and if it is still possible to obtain it for my TL-MR6400 V4 (Hardware version: TL-MR6400 v4 00000001, Firmware version: 1.11.0 0.9.1 v0001.0 Build 200325 Rel.56677n)?

This would be very helpful for resolving my current MTU-related issues.

Thank you in advance.

  @David-TP 

Hi TP-Link Support,

Thanks for confirming. Since the mobile WAN MTU is provider-controlled (1430 in my case), could you please consider adding either:

1) an MSS clamping toggle (TCPMSS = 1390), or

2) an advanced override for WAN MTU,

to avoid PMTUD black holes and fragmentation issues behind the MR6400?

Many LTE CPEs provide MSS clamping to ensure stable TCP connectivity when MTU<1500. This would solve the practical issues without requiring users to add a second router.

Best regards,

Hello,

thank you for your support regarding the MTU topic.

I just wanted to confirm that everything is now working fine — my Galaxy S23 is connecting without issues and the internet is stable behind the TL-MR6400.

At the moment the router is syncing MTU 1358 (MSS 1318) from the mobile provider, and with these parameters all applications are functioning normally.

Best regards