Setting Framed-MTU Attribute

[Luca Bertoncello]

Hi list!

We have a Freeradius 3.0.17 from Debian Repositories to serve the WLAN-Enterprise authentication.
It work(ed) wonderful.

Now we have a second office, connected via OpenVPN. In this office, too, there are devices that need to connect to the WLAN.
We use Unifi to manage our WLANs.

The problem: if the device uses WPA2-Enteprise with password authentication it works via VPN, too.
If the device uses a certificate, the authentication process does not work.

With tcpdump I see:

7	0.326167	10.6.21.10	10.0.21.10	IPv4	1514	Fragmented IP protocol (proto=UDP 17, off=0, ID=ced8)

I already searched in net and I think, I have to set the Framed-MTU attribute to a lesser value.
Current it is 1400 (if I see other packets in the communication).

Problem: I cannot find a way to set it... I already tried to add the attribute in mods-config/attr_filter/access_challenge and in the inner tunnel.
Even is the attribute 1400...

Can someone help me?

Thanks
Luca Bertoncello