Region : UnitedKingdom

Model : TL-MR3420

Hardware Version : V2

Firmware Version :

ISP :

I have a Draytek Vigor 2820 connected to a DSL and providing a DHCP server for my LAN. DSL speed is poor, and I want to connect my TL-MR3420 to the same LAN to give a 3.75G connection. I want the 3.75G connection to be the preferred connection, using the DSL connection as backup.
I'm not sure I'm setting IP addresses correctly. Currently, I have Draytek set as 192.168.1.1, and its DHCP server set to issue addresses from 192.168.1.200 onwards. The
TPLINK is set as 192.168.1.2, with its DHCP server set to issue addresses from 192.168.1.100 to 192.168.1.199.
When I connect my tablet to the TPLINK SSID, it sometimes picks up an address from the Draytek DHCP server, and then uses the DSL connection, even though the 3.75G
connection is available.
I've tried disabling the DHCP server on the Draytek, but find that if I do, fail over from the TPLINK does not work when 3.75G is not available. Should I have the TPLINK and
Draytek on different LAN subnets, for example? If I did, would the TPLINK be able to find the Draytek's DSL connection when 3.75G was unavailable?
I'm a novice with all this, and I'd welcome some advice on the settings I really ought to be using.