Not able to send Vendor specific elements


i want to use Vendor Specific Elements in my Beacon Frames. Unfortunately i wasn't able to succeed.
I am using the Linksys WRT1200AC with LEDE 17.01.4 and the Marvell 88W8864 WiFi-Chip.

I tried the ubus command for hostapd set vendor element but nothing changed in the /var/run/hostapd-phy1.conf or in the beacon frame:

ubus call hostapd.wlan1 set_vendor_elements '{"vendor_elements":"dd08001681caffeebabe"}'

ubus call hostapd.wlan1 update_beacon

Also i tried to modify the /lib/netifd/ to write vendor_elements=dd08001681caffeebabe manually in the /var/run/hostapd-phy1.conf. I managed to do that, but also no additional vendor elements in the beacon frames.

Does someone have an idea, what i am doing wrong? Or could there be a driver problem with my router?

Thank you in advance.