Exact same problem here.

Have used inSIDDer to confirm the 2.4Ghz wireless isn't broadcasting.

Definite problem with this firmware.

Just noticed Draytek has withdraw this firmware for the exact problem we're seeing.

3.7.4.4 is been removed

https://www.draytek.co.uk/download/support/readme_v2860_3744.txt

******************************************************************************
Release Notes for Vigor2860 series

Firmware Version : 3.7.4.4
Applicable Models : Vigor2860, Vigor2860n, Vigor2860Vn, Vigor 2860n+, Vigor 2860Vn+
VDSL Firmware Version : VDSL: 548006_A ADSL, 544401_A, 560816_A
Locale : UK Only
******************************************************************************

[Known Issue]
3.7.4.4 was withdrawn as a WLAN disconnection issue could occur with the Vigor2860n+ and Vigor2860Vn+ WLAN on 2.4GHz. Non dual-band units are unaffected.
[END OF FILE]

3.7.4.5 appeared at main Taiwan ... should be soon at UK download pages, keep an eye

Actually v3.7.8 was released today. It comes in 3 sub flavours: STD, VECTOR1 & VECTOR2 and a long list of new features and improvements. I have bitten the bullet and installed Vector1 (.all version) - simply because I was running the 3.7.4.3 vector version of the firmware previously and it was very stable.

So far so good, certainly both the 2.4 & 5GHz wifi is working.

Release note for latest version says: Support 11ac Wireless LAN (Both 2860n+ and 2860ac use new wireless driver).

Does this means that with latest firmware 2860n+ is now ac compliant?