Each dsl modem needs to have a different IP address from the default one, so on the 2740, set it for 192.168.1.50 and for the 2780 set it to 192.168.1.51 if 192.168.1.1 is the address of the main host router. If the main host router is 192.168.0.1, then just change the 3rd number to 0. Subnet mask should be the same as the main host router.
DHCP, uPnP, firewall and any other options like filters, WPS, and such, need to be turned OFF or disabled. If this works, and after you have set up the addresses and disabled everything, just connect the LAN cable to the LAN port of the modems to the LAN port of the main host router.