My TG788vn v2 is configured with channelwidth=20/40 and sgi=enabled but 40 MHz doesn't work and the Max Rate is limited to 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.
I'd expect "Supported channel width: Channel of any width supported" inside HT Information element.
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)"

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?