Having recently got a 2830 to replace my old 2820 (which stopped working on all LAN ports after a thunder storm). I cannot connect any Android device via the WLAN. Each device (phones, tablets....) shows "authenticating" or sometimes "obtaining IP address" and eventually gives up. I have another wireless AP (Edimax) at the other end of the house, connected to the 2830 through a powerline adaptor and they easily connect to that. Computers (laptops and wireless desktops) have no problems connecting to the 2830 WLAN, it only affects Android devices.
Checking the 2830 while an Android is trying to connect shows it in the station list for a while.

Any suggestions would be most gratefully received.

Firmware 3.6.3 had issues with android phones and are few mentioned at wifi section ... maybe your issue is related
If you have 3.6.3 firmware update to 3.6.4 from UK support pages has the latest but be careful do not download the the sb firmware by mistake ... if you have the + (plus icon) you have the db ... if you don't have many settings to do back from the begin try the *RST* file (will delete all settings)
http://www.draytek.co.uk/support/downloads.html

What is the current firmware? (System Maintenance >> Firmware Upgrade ... or ... system maintenance>>system status... on top is appearing

As for the first issue with the 2820 is it working ?? (have access to WUI but just Lan fails?) or all lights are flashing and is not working atol? (not even access to UI?)

Current firmware is 3.6.4-db - it is a 2830Vn+ and I have reflashed using the rst, but to no avail. The 2820 still works fine but only on WLAN and has no problems with androids connecting.

in this case try the support, maybe they are aware or have some beta

I re-flashed with an older firmware and hey presto, Android devices connect again with no problems. I should have tried this first.

I have since tried some experimentation with the 2830Vn+ and have found the following.

Using firmware 3.6.3_db_232201 - all is working well

Using firmware 3.6.4_db_232201 - Android and IOS devices will not connect wirelessly at all unless all security is switched off. Other devices will connect, however the wi-fi signal drops frequently and the unit requires a re-boot to re-enable it. After re-booting, the Android and IOS problem is still there, but for a short time (5 - 10 minutes), the wi-fi signal drops again.

I will report to support, but for the time being, I'll just stay on 3.6.3