tg788vn v2 and 40 MHz support

[PANJAN]

My TG788vn v2 is configured with channelwidth=20/40 and sgi=enabled but 40 MHz doesn't work and the Max Rate is 144 Mbps instead of 300 Mbps.
In each beacon frame, it sends inside HT Information element "Supported channel width: 20Mhz channel width only", while in HT Capabilities element it sends "HT Support channel width: Transmitter supports 20MHz and 40MHz operation.
The Technicolor support wrote to me:

"...it seems like (as per default) the device is only using the 20Mhz channel but is capable of using the two. It should be possible to activate the 40Mhz channel from the CLI (channelwidth 20/40).
These are the related commands:
:wireless radio channelwidth=20 sgi=disabled (Wireless Speed 130 Default)
:wireless radio channelwidth=20 sgi=enabled (Wireless Speed 144)
:wireless radio channelwidth=20/40 sgi=disabled (Wireless Speed 270)
:wireless radio channelwidth=20/40 sgi=enabled (Wireless Speed 300)
You should contact Forthnet, either because something is wrong with current FW version, or because you need another FW version to support and use 40 MHz."

I contacted Forthnet technical support and they told me that I should be happy with 144 Mbps with an (up to) 50 Mbps connection and the router works fine.
Forthnet is my Internet provider, is only responsible for my Internet and not for my WiFi connection and the provided vDSL router FW, shouldn't block any router's WiFi capabilities.

I have software version
Flash image: 10.5.2.28.0
Active SW: ZV89EN10.52S (10.5.2.S)
Passive SW: ------------ (--------)
Product Name: TG788vn v2
Software Release: 10.5.2.S
Software Variant: EN
Boot Loader Version: 2.0.16
Product Code: 37086010
Board Name: VDNT-W

Does anybody know where can I find a "good" official FW for TG788vn v2?

[Forthnet.Nova.Rep1]

Hello PANJAN,

Thank you for contacting us.

We would like to inform you that the latest firmware release from our service for the TG788vn v2 router, is 10.5.2.S-EN__VDNT-W__1.0 that as you mentioned, you have it already.

Although, we will make sure to inform our firmware development department about the above observations.

We remain at your disposal.

Alex, Forthnet Support