Setting up an IPSec(?) VPN for a Small Business

I work for a brick-and-mortar store as a technician and I've been attempting to setup a roaming VPN for a small business on-site.

The gateway is a D-Link DSR-500AC (one of the only gateways/routers that matched all the criteria he required that we sell). I followed this guide to set it up (with increases in the security algorithms): http://files.dlink.com.au/Products/DSR-500AC/REV_A/SetupGuides/How_to_setup_L2TP_VPN_Service_in_DSR-1000AC_500AC.pdf

I have the L2TP/IPSec server VPN setup. The only way I could get it to connect correctly was to set an L2TP secret AND an IPSec pre-shared key, with a username and password. The guide above mentions not using an L2TP secret. Windows' native client only allows setting the IPSec pre-shared key. So the first question: What's the difference between the two keys/secrets? Everything I can find online only ever implements one or the other, not both.

As of yet it works fine on Android's native client over LTE. On WiFi, it only worked once I enabled L2TP and IPSec pass through on my home router. So the second question: Is there any way to deal with the pass through issue for the employees that will undoubtedly not have VPN pass through on their home routers?

Thanks guys!