Setting up Opnsense with router

[Vilhonator]

Hi, I am trying to get my OPNsense to work with my router but can't figure it out because obviously both have totally different GUIs and descriptions to different features.

First of all I have Asus RT-AC66U B1 router and Zyxel GS 1900-8 10/100/1000 Mb/s Manageable switch which is not in use since I figured I wouldn't really need it unless I would have to setup VLANs etc.

My problem is that every time I connect my computer and PS4 etc to router which is connected to OPNsenses LAN port, I get warnings about having "strict NAT", and I can't open ports to anywhere even when I set my router to access point mode (which will disable firewall, NAT and connects the router to first network it can find which is OPNsense) and also network speed is sometimes much slower than my average speeds.

I've tried basic routing and from my router everything works fine but OPNsense can't traceroute or ping to my router (in other words it doesn't find my router at all) yet I am connected to the internet and everything except port forwarding works just fine.

I have watched all guides I can find about how to create port forwarding, routing, 1:1 nat and even virtual IPs on OPNsense and none of them works (few of them even locked my out from OPNsense and had to re-install the whole OS since I couldn't login as root on console).

My question is this, is it possible to have a router with different internal network taking care of port forwarding as long as those ports are also open on OPNsense and if so, how the heck it is done, or would it be possible to use my switch as kind of a gateway for both opnsense and router and still be able to use OPNsense as a firewall for the router as it happens, if I do that all 3 devices will get their own public IPs in different networks?

I also have 3rd ethernet card which I could install on OPNsense, but would it require static IP or anything else or would it do things automatically as long as I set router to Wireless router mode?

[Snowmanut]

Newbie myself but you might want to see other posts on this topic.
https://forum.opnsense.org/index.php?topic=8812.0
or this
https://forum.opnsense.org/index.php?topic=3521.0

Side note. Why not have Opnsense do all the routing and just make the AC66u just be a wireless access point?