Trouble setting up L2TP VPN with IPSEC on EdgeRouter-X in DMZ (behind an unavoidable ISP-router)

Hi everyone

I have some trouble setting up L2TP VPN with IPSEC. I want to be able to 'VPN home' to my ER-X-network and ssh into my workstation(s) and similar tasks.

I'm new to networking, and I have mostly bought my ER-X to challenge myself and hopefully get some exciting new advantages out of it. Unfortunately do my ISP provide an Fiber-modem/router-unit, that I cannot exchange for just a modem, or even set in bridge mode. I guess this VPN problem is related to this rather complicated setup, where the ISP-router has a dyn-IP and a dyndns-setup. My ER-X's WAN is then connected to the ISP-router where it is set in a DMZ. When I allow port 80,443 in ER-X local firewall I am able to reach the GUI from the outside. 

I have tried to following the complete guide from https://help.ubnt.com/hc/en-us/articles/204959404-EdgeMAX-Set-up-L2TP-over-IPsec-VPN-server. But when trying to connect with i.e. my android it will not connect. I'm a little confused wether to use the dyndns option or the fixed wan-ip option (does it make any difference?) Here is my ER-X l2tp vpn configuration ('hugo' and 'toto''s are just placeholders oc.) :

$ show vpn l2tp
 remote-access {
     authentication {
         local-users {
             username hugo {
                 password toto
             }
         }
         mode local
     }
     client-ip-pool {
         start 192.168.10.10
         stop 192.168.10.100
     }
     dhcp-interface eth0
     ipsec-settings {
         authentication {
             mode pre-shared-secret
             pre-shared-secret toto
         }
         ike-lifetime 3600
     }
     mtu 1024
 }

I have the following serial setup of the two routers, if that can be of any help:

--- Internet fiber-connection --->
{ "ISP-router"
WAN-ip: [dyn-IP]

local IP-adress: 192.168.0.2

DMZ: 192.168.0.1

eth1: 192.168.0.1 } 

---eth1 connected to eth0---> 

{"ER-X"

eth0(WAN): 192.168.0.1/24

eth4(WLAN): 192.168.4.1/24}

--- eth4 connected to eth0 --->

{Unify AP AC-LR (working fine!)} 

If I can provide any other information that can give clarrification, please say so. 

I'm completely lost in explaining why I can't connect to the l2tp vpn service on the ER-X when I'm able to reach it's webserver(GUI). Should the IP-pool of the vpn-clients be within my existing dhcp pools for WLAN, or could I have it seperate as in the above configuration?

Thanks in advance. Comments on my general configuration is welcome as well, since I'm just trying to learn as much as possible here.