Add a wireless router to supplied router?

[#1]

A relative has a SpeedTouch 516v6 1-Port Router, and wants to add multi-user connectivity.  I'm trying to add her TP Link TL-WR741ND wireless router in series.  I've been trying to get this working all day without success.
I've tried to find some guides on how to do this by searching but nothing found.  The only relevant thread I found suggested disabling the DHCP server on the additional router, but this made no difference.
Another frustrating part of this is I've tried and tried to login to the SpeedTouch 516v6 but it won't accept the default password, not even after a reset.
Any help appreciated.

[#2]

Hi abssorb,
The old 1-port sometimes used Admin or Administrator with the capital A's
Jojo

[#3]

Have you tried Administrator as the router user name with the password field left empty (no password)?

[#4]

From a quick skim of the TL-WR741ND manual it doesn't look as if the TL-WR741ND will route DHCP requests from the LAN to the WAN (even if the WAN is set up as Ethernet) so that suggests you have two options
You could connect the TL-WR741ND  WAN port to the speedtouch and use the TL-WR741ND DHCP for the PC's connected to it. That way you would get have four available LAN ports on the TL-WR741ND. Setup should be fairly easy, however you will not be able to see the  Speedtouch from the LAN. 
The alternative is to connect one of the TL-WR741ND LAN ports to the speedtouch (the ports on the TL-WR741ND  seem to be autosensing, so you shouldn't need a crossover cable), disable DHCP on the TL-WR741ND  and ignore the WAN functions. It will need setting up a bit more carefully I suspect, and you will loose one of the LAN ports, but you will be able to see the speedtouch from the PC's. (Logging in to it is a different issue!)
For the first approach this is what I would do - but I am only guessing I have neither a speedtouch 516 nor a TL-WR741ND
Connect a PC to one of the TL-WR741ND LAN ports, DO NOT connect the TL-WR741ND  to the speedtouch.
Restart the TL-WR741ND , reboot the PC ( in that order).
You should have access to the TL-WR741ND  control panel on 192.168.1.1 Username and password are both admin
Set the WAN connection type to Dynamic IP
On the LAN screen set the LAN IP address to 10.0.0.1 subnet 255.255.255.0
Enable the DHCP
Set the Start IP Address  to 10.0.0.100
Set the end IP address to 10.0.0.200
Set the default gateway to 10.0.0.1 (this may not be required)
Switch off TL-WR741ND and PC.
Switch on TL-WR741ND  and reboot PC
You should now be able to see the TL-WR741ND admin interface at 10.0.0.1
Connect the TL-WR741ND  WAN port to the speedtouch and Renew the WAN setup
You should get a 192.168.1.X address from the speedtouch and a default gateway of 192.168.1.1
The PC should now be able to see the internet
You might need to transfer the default DNS server addresses from the WAN page to the  DHCP page
You now have a private 10.0.0.X network linking the PC's to the TL-WR741ND, and a separate private network  192.168.1.X linking the TL-WR741ND  to the speedtouch.
I leave wireless setup to you.
The other way
Access the TL-WR741ND as steps 1-3 above
Set the WAN connection type to Static (I don't think this matters but!)
On the LAN screen set the address to  something like 192.168.1.220 subnet 255.255.255.0
(I am presuming that this is  outside the Speedtouch's DHCP allocation or at least unlikely to be issued)
Disable DHCP
Switch off TL-WR741ND & PC
Connect one of the TL-WR741ND LAN ports to the speedtouch
Connect the PC to another of the TL-WR741ND LAN ports
Switch on TL-WR741ND and PC (in that order)
You should now have access to the Speedtoch config pages at 192.168.1.1
You should have access to the TL-WR741ND configuration pages at 192.168.1.220
You should have access to the internet from the PC
I am not as sure about Wireless in this setup - I presume that the TL-WR741ND will pass DHCP requests over the wireless link to the speedtouch but you might need to play with the default gateway settings on the WAN screen ( set it to 192.168.1.1?)
Let me know if this works - its really a thought experiment.
Unless you want the wireless capability, it would be quicker and easier to invest in a cheap (£10-15) switch from E-bay.