You technically have 2 routers. The OPPO device acts as a router. The manual seems to be hard to find but that appears to be a wifi6 router. Hard to say but it appears to pretty much be the same as your tplink at least as far as wifi support is. Why do yuo feel you need the tplink
If it is some easier configuration thing then maybe the tplink might give you some benefit.
So you have 2 basic options. If it just you want different wifi I would run the tplink as a AP. This will just use the wifi part of the tplink. The other is to run in router mode.
When you run in router mode you are best off running the ISP router as only a modem/bridge mode. Since I can't find the manual it is hard to say if you can do that.
You should be able to just run router behind router and pretend the ISP router does not exist. Most times this just works. What I suspect the problem is both routers are using the same range of IP addresses. The TPLINK router will not allow the wan ip (which it gets from the OPPO) to be in the same range as the LAN network on the tplink. You have to change it on one of the devices. Since I can't find the manual this may not be the issue but should be easy to check on the tplink and see what it gets for a wan IP
Thanks. Initially tplink had a different IP range 192.168.1.1. Then I changed it to AP mode, and also changed the IP to 192.168.0.2 to be in the same series of the Oppo router (initially tplink IP was 192.168.1.1).
Now the internet works fine through wifi, but any device connected through tplink's LAN doesn't work.
Additionally, Oppo has a LAN and WAN/LAN ports. When I directly connect an Android TV or hikvision CCTV DVR to any of these ports, internet doesn't work at all. But when I connect the tplink to Oppo via LAN cable, any device connected to the tplink through wifi has internet (but not though LAN).
I guess the issue here is on the LAN side. But I am not sure what additional configuration I need to do there.
Also, an interesting finding: as mentioned earlier, when I connect the tplink to another 4G CPE device, everything works fine, including the Hikvision CCTV thru LAN. And then once it started working, I simply disconnected TPlink's LAN cable from the 4G CPE, and plugged it on to the Oppo, and everything remains working fine until I restart the wifi.