Short answer

TP-Link needs to be itself configured for the IP address it was assigned, i.e. in Network > LAN submenu set 10.0.0.20 as the address.

Long answer

(Disclaimer: I am not a network professional, please excuse/correct improperly used terms.)

I ran into the same situation, and the problem I had was a mess of the address setup initially: main router as 192.168.1.1, the TP-Link as 192.168.0.1 router+AP, and I needed the main router to become the 192.168.0.1 and TP-Link working as just the AP while still being able to access TP-Link's administration.

The goal to achieve is to have the TP-Link as AP on the same network segment but with different address than the main router and outside its DHCP range (e.g. main = 192.168.1.1 with DHCP 1.100-1.199, TP-Link AP = 192.168.1.2), plus properly configured itself.

Setup

WAN <---> main router <---> TP-Link router 
            ^                  ^    
            |                  |
            v                  v
          computer           computer

The main router has DHCP enabled and you can access its administration. On TP-Link, the cable to main router is in LAN port, not in the WAN port. The computer (e.g. notebook) is connected via Ethernet i.e. wired, not wireless.

Procedure

1. Note or decide on the main router's IP and DHCP address range (e.g. 192.168.1.1 and 1.100-1.199), based on your goal.
2. In TP-Link's admin pages, select Network > LAN submenu, and provide an address for the TP-Link according to the goal, e.g. 192.168.1.2 . Reboot. Check that you can connect to the TP-Link administration; if you changed it's segment part of the address, you may first need to reconfigure the static address of the computer's network connection.
3. Switch off DHCP on TP-Link (menu DHCP > DHCP Settings: DHCP Server: disable). Reboot. (Attempt to access it's administration may fail now. Keep going.)
4. (if needed) Reconfigure main router to have IP address and DHCP range according to the goal (e.g. 192.168.1.1 and 1.100-1.199). Reboot.
5. (optional) Set the static address of TP-Link in main router's config to the one entered in step 2. Switch off firewall etc. in TP-Link's config.

At this point, you should be able to access main router (on 192.168.1.1) as well as TP-Link (on 192.168.1.2, acting as AP only), and devices connected by WiFi via the AP should get correct addresses from main router's DHCP range.

Debugging

If a device's administration is not accessible, it can be because of these reasons:

• Conflicting network segment / IP address configuration (e.g. main 192.168.1.1, TP-Link 192.168.2.1) -- disconnect the cable between the two devices and connect your computer directly to the one you need to configure, as shown in Setup; once problem is solved, reconnect again.
• Misconfigured / disabled DHCP of the device, thus the notebook cannot get it's IP address and communicate -- if you know the IP / segment of the device, set the notebook's network connection address as "static" on the same segment (e.g. 192.168.1.111 for a main on 192.168.1.1)

If any of the above does not help, or if you cannot determine the device's segment / IP address, then factory reset of the device will configure it to a known default IP.

So the first problem I saw was it seems you have the definitions of external and internal IPs confused. But that shouldn't keep us from helping you. :)

So First off, you cannot access your router through your modem by using 192.168.0.1 as this is assigned to the router by itself. The clients will be able to access it in this way, but not the modem, or anything connected to the modem, because it doesn't have a DHCP entry with that address.

That aside, the reason you might not see the configuration screen for your router is because the port is locked down. As far as the router knows, your computer is a random computer from the internet, as such the firewall blocks most ports from connecting. It could also be that your port 80 is being used by a different service.

In any case, the only way to access your router would be to open port 80 in port forwarding settings and have it point to 192.168.0.1 (in router settings). This should do what you want. Here is a quick guide.

• Open your router's configuration
• Click Advanced Settings on the left hand side
• Click NAT
• Click the Add button
• Select the Custom Service radio button
• In the Name field type something like Router Config
• Enter 192.168.1.2 as the server IP.
• For all External and Internal port fields type 80.
• Click apply and save.

Just note, I was unable to find the exact model you specified as your router. So the settings or names may change. Regardless you are looking for NAT or Virtual servers as a setting option. Most of the values should be the same from there.

You should now be able to access your router from your desktop connected to the modem. (If not try restarting the router)

EDIT:

The method above should allow you to see the configuration screen from the clients of the modem. If NAT is unavailable, you need the Virtual Server option.

So you want to share files between systems... This procedure becomes more complicated at this point...

First thing you will need is a FTP server hosted by one of your systems. You can find many a many tutorial online. For Windows, Filezilla works for a server, for Linux systems you have to be a bit more in depth.

I won't post a guide on setting one up, due to there being many ways and systems across your network. And lack of information on a potential server.

However, once you get a FTP server running on your "server" you just have to port forward it with the same method above. However changing the port from 80 to 21 (if you keep the default).

Hope this Helps!