I am having some issues when using my Zyxel 5G modem in PPPoe mode connected to my OWRT router. The internet signal gets interrupted and having stupid delays quite often whereby the speed is great 500/50 Mbit. Adjusted MTU - but not fully solved.
This modem can also work in router mode (Zyxel NR7302) and I would like to test if this maybe solves my issues.
Maybe you can support me in doing so?
I would like to have the functionality Wireguard (in/out) and DHCP on my OWRT router (Cudy WR3000s). I might need to use port forwarding as well.
Sure I do need to change the OWRT router in client mode.
What else do I need to change, I have heard about double NAT and route but I don´t know how to use and apply that or what else is required or recommended.
Can you help me here please?
if you connect your openwrt router using the default config and wan port, you'll double NAT.
if you don't want it, use it as https://openwrt.org/docs/guide-user/network/wifi/wifiextenders/bridgedap.
it doesn't stop it from being a wireguard server host.
I think we previously concluded you were CGNATed, how is the incoming WG traffic supposed to reach you ?
OK, this setup I am using already for further APs connected to the LAN.
Today the first OWRT router acts as DHCP server connected to the Zyxel modem/router
in PPPoe mode.
I would like to change the Zyxel to router mode and use the first OWRT still as DHCP server (and Wireguard later on if possible CGNATed).
But I don´t understand where and why double NAT will happen and why this setup will avoid this, can you bring a bit more light inside? 
1st NAT is by your carrier, the 2nd one is when the traffic is passing your router, unless the Zyxel is the only router.
So that means when all APs behind the Zyxel are in Dump AP mode I won´t have double NAT, that sounds so simple?
I still can deactivate DHCP on the Zyxel as router and run DHCP on the first OWRT AP, as of today, that isn´t described in the guide?
And I need to change WAN to LAN plug on my first OWRT AP, right?
The 1st NAT is between your router and the public IP, done by the carrier, you won't be triple NATing.
It's not, but you only have to leave dnsmasq running.
Yes.
Sorry to ask
dnsmasq, is it in standard luci config available and what do I need to do there?