The link in your post seems to have some extra characters hidden at the end. Here is the kb article link
https://forum.peplink.com/t/configure-ipsec-to-a-juniper-ssg-firewall/8304
This kb outline setting up a site-to-site VPN connecting a private network at peplink to a private network on the SSG.
Peplink LAN Network 192.168.2.0/24
Juniper SSG LAN Network 192.168.1.0/24
When completed local LAN SSG clients on 192.168.1.0/24 will be able to communicate with services published by peplink on 192.168.2.0/24.
There is no internet default route on this particular vpn setup. If they are providing internet via this vpn peplink is likely giving you a proxy server address in the 192168.2.0/24 subnet to configure for your client devices to use. The only connectivity from this vpn setup is between those two private networks.
You won't need any dhcp on the untrusted interface to your clients they will still use the local lan dhcp as they do now.