That link I posted to the user manual is definitely for the R7100LG. The AC1900 bit is just the wifi speeds specification (combined wifi speeds)
I'm well aware that this is a standard router with LTE built-in, you should try to see first if you can get L2TP working over the router's own LTE connection - it may work or it may not. But if it does, its a lot simpler than having a 2 box solution.
Did you follow the steps below? At step 6 you need to select L2TP.
Specify an Internet Connection That Uses a Login
To view or change the basic Internet setup:
1. Launch a web browser from a WiFi-enabled computer or mobile device that is connected to the network.
2. Enter http://www.routerlogin.net.
A login window opens.
3. Enter the modem router user name and password.
The user name is admin. The default password is password. The user name and password are
4. Select Internet.
5. For the Does your Internet connection require a login? setting, select the Yes radio button.
6. From the Internet Service Provider menu, select the encapsulation method: PPPoE, L2TP, or PPTP.
7. In the Login field, enter the login name that your ISP gave you.
This login name is often an email address.
8. In the Password field, type the password that you use to log in to your Internet service.
9. If your ISP requires a service name, type it in the Service Name (if Required) field.
10. From the Connection Mode menu, select Always On, Dial on Demand, or Manually Connect.
11. To change the number of minutes until the Internet login times out, in the Idle Timeout (In minutes)
field, type the number of minutes.
This is how long the modem router keeps the Internet connection active when no one on the network
is using the Internet connection. A value of 0 (zero) means never log out.
12. Select an Internet IP Address radio button:
• Get Dynamically from ISP.Your ISP uses DHCP to assign your IP address.Your ISP automatically
assigns these addresses.
• Use Static IP Address. Enter the IP address, IP subnet mask, and the gateway IP address that
your ISP assigned. The gateway is the ISP modem router to which your modem router connects.
13. Select a Domain Name Server (DNS) Address radio button:
• Get Automatically from ISP.Your ISP uses DHCP to assign your DNS servers.Your ISP
automatically assigns this address.
• Use These DNS Servers. If you know that your ISP requires specific servers, select this option.
Enter the IP address of your ISP’s primary DNS server. If a secondary DNS server address is
available, enter it also.
Specify Your Internet Settings
14. Select a Router MAC Address radio button:
• Use Default Address. Use the default MAC address.
• Use Computer MAC Address. The modem router captures and uses the MAC address of the
computer that you are now using.You must use the one computer that the ISP allows.
• Use This MAC Address. Enter the MAC address that you want to use.
15. Click the Apply button.
Your settings are saved.
16. Click the Test button to test your Internet connection.
TalkTalk Business Fibre 900 -- Netgear RAX200