How to make ISP Router (used just for the VoIP functionality) work behind OpenWrt Router?

[jakklops]

Hello guys, I need your help once again . For the past few days I've been trying to setup my ISP Router to work behind my OpenWrt Router (FRITZ!Box 7530).
I need the ISP Router only to be able to use my Analog Phone, so I'll just use it as an ATA.
My ISP Router is a Zyxel VMG8828-B50B, it's a branded Router from my ISP (WINDTRE), it should be very similar to the Zyxel VMG8825-T50K.
This is my current setup:

What I did to make it work is as follows:

1. I went in the WAN Interfaces config of the ISP Router and deleted (maybe this wasn't necessary, but I did anyways just in case) all the WAN Interfaces except for: ETH, ETH_VoIP. I then switched the Encapsulation of those 2 interfaces from PPPoE to IPoE;
2. I went in the Network -> Interfaces -> Devices config of the OpenWrt Router and made 2 VLANs (lan4.835, lan4.836). I then edited the "br-lan" Bridge device and on the "Bridge ports" option I removed lan4 and added lan4.835 and lan4.836;
3. I then connected the ISP Router WAN Port to the OpenWrt Router LAN4 port.

Here are the screenshots: Current config

It "kinda works" but I've got a few problems:

1. The ISP Router Internet led is green and if I go on the admin panel I can see it works by going in the Diagnostic tab and pinging 8.8.8.8, the problem is I can't ping 192.168.0.1 (OpenWrt Router) and also the DNS isn't working, so pinging google.com results in unreachable host. Also one strange thing is that if I do a traceroute 8.8.8.8 I can see 192.168.0.1 as the first hop, but if I ping it directly it doesn't work. If I, however, also connect one of the lan ports of the ISP Router to one of the lan ports of the OpenWrt Router (so ISP WAN <-> OpenWrt LAN and ISP LAN <-> OpenWrt LAN) everything works;
2. The ISP Router Phone led is orange (unregistered) unless I do the same as before (connecting also one lan port), I think the 2 problems are basically the same because otherwise it can't resolve voip.windtre.it.

So my questions are:

1. How should I fix it? What are the configuration changes needed to get it to work properly?
2. What I did until now is correct? I mean, I'm referring specifically on point 2. of the first section (the 2 VLANs and edit of the br-lan Bridge device). Should I make a separate interface, let's say "voip", and put the 2 VLANs there and not in br-lan? Should I also put the Switch port lan4 in there or not?
3. Should those 2 VLANs be in the "lan" firewall zone or "wan"? I think they should be in the "lan" one like I just did, but just wanted a confirmation;
4. On the OpenWrt Router I already had the 835 VLAN on the wan interface (dsl0.835) for the Internet connection, but I also made another 835 VLAN for the LAN4 port (lan4.835), is this right? What would be the correct way of doing it?

I need some clarification, If someone could explain me what I should do I'd be very grateful. Thank you.

[mk24]

If the ISP router's LAN is also 192.168.0.0/24, that entry in its routing table will keep it from using the same IP subnet for the default route via your router. You will need to change one of the LANs to something else.

[jakklops]

That makes so much sense! As soon as I changed the IP subnet of the ISP Router it just worked flawlessly. Thank you so much!

Would you be so kind to also check the last 3 questions in the first post? Just to make sure I did everything correct. Thank you again!

[jakklops]

Ok, so I did a little bit of research and regarding my last 3 questions this is what I concluded:

1. What I did is correct, but I don't have to put the Switch port lan4 in there too, just the 2 VLANs is the correct way of doing it. Also I put them in a separate ("untrusted") interface isolated from the rest of the network together with other iot devices, where nothing can to talk to each other and they can only go on the Internet;
2. The "lan" firewall zone is correct, but I made a new zone for the "untrusted" interface where I put the 2 VLANs;
3. Yes, that's correct.

This is the guide I read to make the "untrusted" interface and firewall zone: https://openwrt.org/docs/guide-user/network/wifi/guestwifi/configuration_webinterface

Thank you again @mk24 for helping me!