Distribute the correct MTU to the clients

My Setup:

┌             ┐    ┌          ┐    ┌      ┐    ┌         ┐
│ LTE-modem   │───►│  Router  │───►│  AP  │───►│ Clients │
│ bridge-mode │    │ Open-WRT │    │ Dumb │    │         │ 
└             ┘    └          ┘    └      ┘    └         ┘

My clients get the value 1500 as the value for the MTU (checked with windows-command netsh interface ipv4 show interfaces.
I then played around with the following ping command: ping google.com -f -l 1500
This resulted in the message that packets having to be fragmented.
I gradually decreased the packet size until I got no more fragmentation at the value 1444.

Am I correct in assuming that this is the correct MTU for my Internet connection (or is there some offset that needs to be applied here)?
Am I correct in assuming that values above this will cause fragmentation on IPv4?
Is it correct that IPv6 doesn't mind this quite so much, since the clients themselves determine a reasonable value for the MTU via Path MTU Discovery?

If I am correct, I would like to distribute the correct MTU value of 1444 to the clients as well. The only question is where do they get their value from?
I have set the MTU value for all upstream and downstream interfaces on the router to 1444. However, the clients set an MTU of 1500 (checked with netsh interface ipv4 show interfaces).
In the DHCP6 settings of the downstream interfaces I have set the option ra_mtu '1444, in the DHCP4 settings also the option 26,1444.
The client still receives the ICMP6 router advertisement with an MTU of 1500. The DHCP6 replies do not contain any information about the MTU. The same applies to the DHCP4-ACK, which is probably due to the fact that the client does not request the value 26 in his DCHP request.

The router itself receives the MTU value 1500 from the modem via router advertisement (no MTU value in the DHCP6 messages). However, this value should be the wrong one according to my tests - or have I completely misunderstood something?

So where does the RA message get the value 1500 from and how can I overwrite it so that all clients get the correct value 1444?
Or I have quite misunderstood something regarding the MTU. In that case it would be nice if someone could explain this to me.

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  • Optimize MTU on the WAN interface.
  • Make sure that MTU fix is enabled on the WAN zone.
  • The rest should be automatically taken care of by PMTUD.
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Ok with these steps my MTU should have the value 1472 (1444+28).
The MTU fix is already set in the wan zone.
And the upstream interfaces wan and wan6 have the option mtu '1472'.

But my client still set his MTU to 1500 (as shown with command netsh interface ipv4 show interfaces)

So is this a problem or not?

No problem, it should be fine.
You don't need to reduce MTU for LAN-to-LAN commutication.
And PMTUD tells your clients to decrease MTU for LAN-to-WAN traffic traversing your router.
You can verify it with tracepath from a Linux host.

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