IPSec Tunnel is connected to other site (for site-to-site vpn). How do I authenticate with l2tp?

I am trying to configure a site-to-site VPN with my Sophos XG and another non-sophos firewall. I am able to set up the IPSec tunnel but I cannot figure out how to authenticate using l2tp. I originally wanted to do an SSL site-to-site VPN but it appears that I cannot import an ovpn configuration file into the SSL vpn settings (it only takes .apc or .epc format). I can see the l2tp (remote access) tab in the VPN section of the XG config but I can't seem to use it to authenticate over my IPSec tunnel. At the other site the logs show that the tunnel is up but the l2tp session is inactive. Any help/advice is appreciated! Thanks!