Similar issue here on a 2860n Plus - it appears to be on the 5g side - If connected to SSID 1, SSID 2 signal starts dropping in and out (Can be seen on wifi timed graph)...

Update from DrayTek support today:-

"I've tested this issue and can confirm that if device is connected to
SSID1 on 5GHz spectrum SSID2 is not seen and is not available from
this device unless this wi-fi client is disconnected from SSID1 and then
can re-connected to either SSID1 or SSID2.

During my test, while the wi-fi client had been connected to SSID1, I didn't have any
problem with connecting another wi-fi client to SSID2 and
connecting to the router.
Therefore, SSID2 was broadcast properly, but hadn't been seen by devices already
connected to SSID1 (and vice-versa).

I've been contacted by phone support engineer Pawel who confirmed my
findings."

I'll report this to 3rd line support, so that will be investigated
further and I'll keep you updated."

Any progress on this. I have come accross the same issue on 2850 and sbs 2008. It is particular to windows 8 machines only every single one of them on wifi has the same issue. DHCP is obtained but cannot ping the router

vectorbs wrote: Any progress on this. I have come accross the same issue on 2850 and sbs 2008. It is particular to windows 8 machines only every single one of them on wifi has the same issue. DHCP is obtained but cannot ping the router

And you are sure that ICMP ping is enabled?

Yes offcourse. As I said it is only windows 8 machines. There are a number of Win 7, macs and other mobile devices connected without issues.

Windows 8 machines do received DHCP fine from SBS 2008 but after that the connection stays limited. I have even formatted one of the machines to test on bare bone without AV/firewall or any other software with the same result.

The windows 8 machines connect to other routers without any issues and they are of different makes with different wifi cards. At times there would be a few replies from the ping but would then drop off completely as unreachable.

I have factory reset the router and have redone the configuration. I have also changed the channels. I have changed from b, g, n to b, g for the wifi on draytek. The only time it worked flawlessly was when i changed it to b,n 5Ghz. Seems that windows 8 machines are struggling on 2.4 GHz.

They can however connect fine over lan but not all of them have the lan cards and access to lan ports where they are being used.