Incoming connections from the internet through ISP router (internet/phone/TV) to LAN do not work

Pinhead05

I have this problem:
The ISP 2 router in my above configuration connects to the internet and computers in LAN1 can connect to the internet through it.
However, although I have configured port forwarding on the ISP 2 router to 192.168.3.101 (called DMZ on this router) no connection ever reaches LAN1, although the firewall on the ISP2 router says that the rules that grant access have been applied many times. Direct port forwarding to the email server LAN1 address do not work either.

On the Zywall USG 100 I have NAT forwarding for WAN1 and WAN2 and a firewall rule that allows connections from WAN (which includes WAN1 and WAN2) to the email server on LAN1.
This works for WAN2 but not for WAN1.

I set up a vlan that includes WAN1 and LAN1 so I can now access computers on the ISP 2 LAN (192.168.3.nnn) from LAN1 but I still cannot access LAN1 from either the internet or the ISP 2's LAN.

I tried a bridge for WAN1 and LAN1 but that messed up everything and I lost access to the Zyxel USG100 and the internet.

What do I need to configure on the Zyxel USG100 to get the NAT port forwarding through WAN1 to work?
The goal is to have access from the internet to my email server through WAN1.

Thanks for any help.

#Biz_Security_May

Blabababa

@Pinhead05
From the topology it seems that in WAN1 it's a double NAT scenario, are you able to capture the packet from WAN1 and identify what kind of packets are dropped on USG100?Moreover, in this case port mapping  will be required on USG100 as well, had you configured it?

Here I found a FAQ example for your reference.
https://businessforum.zyxel.com/discussion/1171/how-to-allow-public-access-to-a-server-behind-zywall-usg