Maybe there is a compatibility issue between your modem and MR3420.
MR3420 V1 support 4-address MAC Address format.
When it works in WDS mode, it requires that the main router also supports 4-address MAC Address format instead of 3-address MAC Address format, so that it can WDS the main router successfully.
Follow the guide as below to check your settings. If there is nothing wrong, you should confirm with your modem's support.
http://www.tp-link.com/en/faq-227.html

Thanks for the reply.I proved that this does indeed work as this morning I tried the very same thing connecting the TL-MR3420 to a Draytek Vigor 2850Vn router and it worked perfectly. The issue must be something on the Vodafone ADSL router..I will have another look later on this evening and hopefully I'll figure it out.. Must be a DHCP issue I guess...

I semi got this working tonight but still not very well.I successfully got the TL-MR3420 talking to the Vodafone router and letting the Vodafone router control DHCP for IP addresses. I've the SSID's and passwords the same with the same standards but the issues arise when I walk around and try to wirelessly switch between both routers.

Initially when I boot everything up I'm able to connect wirelessly to the TL-MR3420 router and I've Internet and everything is working fine. If I then walk down to the other side of the house to where the Vodafone router is the first problem is that the laptop doesn't automatically connect to the stronger WiFi signal of the Vodafone router, it holds maybe 1 bar signal strength.

If I switch the wireless off and on on the laptop then it will then connect to the Vodafone router and again everything is fine and I've internet access etc.The problem arises when I go back up to the other side of the house again where the TL-MR3420 router is. I am connecting to the router but I don't get internet access. It's like the router get confused. If I power everything down and back up again then it will re establish internet connection.I'm really confused..

Any ideas why:

1. It doesn't automatically switch between the routers?
2. Why when I come back to the TL-MR3420 router it completely loses internet access?

Thanks again for any help.

M Diamond wrote

Maybe there is a compatibility issue between your modem and MR3420.
MR3420 V1 support 4-address MAC Address format.
When it works in WDS mode, it requires that the main router also supports 4-address MAC Address format instead of 3-address MAC Address format, so that it can WDS the main router successfully.
Follow the guide as below to check your settings. If there is nothing wrong, you should confirm with your modem's support.
http://www.tp-link.com/en/faq-227.html

I am having a similar issue with trying to connect TL-MR3420 to Archer C9 (AC1900) via WDS bridge.

How do I find out what MAC address format the C9 supports.
It works the other way around when connecting the C9 to the TL-MR3420 via WDS bridge.

Isn't this weird and unusual?

C9 supports Three-Address Format WDS.
If MR3420 works as the main router and C9 works in WDS mode, it should be OK.