Take both routers and connect the lan ports. Disable DHCP on one of the routers, assign the second router a static IP in same network as the first... ie if your main router is 192.168.1.1 make the second router 192.168.1.2. I would make dsl be your main router and the default.
Now the tough part. You need to manually add route command to each machine for each location you want to use your LTE.
For example ROUTE ADD 18.104.22.168 mask 255.255.255.255 192.168.1.2
It will more than likely take you a few tries to get the list of ip addresses. You will have to experiment since some sites have multiple IP If you were to send one out the DSL and the other out the LTE the site will see it as 2 different external addresses flag it as a attack. To make matters worse you have to reissue all the route commands every time you reboot...put them in a .bat file run at logon is best.