cannot access web gui

[router]

I just installed opnsense on an old laptop, my plan is to use the laptop as a router / firewall. I logged in with root and opnsense, clicked 1) assign interface for my lan, the lan is plugged into my router, my tplink router ip is 192.168.1.2, if I try option 7 ping host it does not get a reply from the tplink router. and I cannot open opnsense gui from my other laptop on the same network. on the install screen it says for GUI goto the following url OPNsense.localdomain but that doesnt work. where am I going wrong? thanks

[router]

ps. even if I try option 2: set interface ip address, after completing the process it says I can access the gui on that ip for example 192.168.1.150 but when I open that ip on my other laptop it does not load.

[cookiemonster]

As long as the device has only one interface, you'll only have one network and all will be reachable only on it.
Does the laptop only have one interface, normally a single RJ45 on the side? If yes, then you will have no route to get out of it. Perhaps if you describe the physical setup it'll be easier to advice.

[router]

yes it has a single rj45 on the side and it is plugged in to my tplink router which has internet, my other laptop is also plugged into the same tplink router, the opnsense laptop has built in wifi and I have a couple of usb wifi dongles which I could use if needed. do I have enough hardware to get this setup as a router? basically I thought it was enough just to connect the opnsense laptop to the tplink router and then access it with my other laptop to configure it via the gui at an ip address I specify, I just wanted to configure a new wifi on dhcp from the gui. thanks

[router]

ps. If needed to get this up and running I have a usb to rj45 so the full setup could have 2 x rj45s and 2 or 3 wifis.

[router]

I just grabbed my usb to rj45 but its usb c and the old laptop doesnt have a usb c socket so until I get a usb c to usb adapter could this be configured to work on a single rj45 along with the built in wifi and another usb wifi if needed? thanks

[cookiemonster]

A few pointers.
A router "routes" networks. Normally that is public on your WAN with private on your LAN.
This is why you need one interface for each normally. A workaround is what is termed "router on a stick". Google it. You need VLANs to split the traffic in the same interface but you need a managed switch.
For learning purposes a usb-to-rj45, dongles, etc. can sometimes made to work but for permanent setup it will give you a lot of headaches.
If you're plugging a router (OPN) into another router (tplink) you'll be having two devices doing the same duties unless you disable some at which point is very different setup. Google double NAT.
For what you want, daisy chain routers you might want to see this article https://homenetworkguy.com/how-to/use-opnsense-router-behind-another-router/

[router]

Thanks for all the info but as a complete newbie it is way above my head and will take a while to understand.

Getting down to the very basics, is it possible to plug my opnsense laptop into my tplink router and then access the opnsense GUI from my other laptop which is also plugged into the same tplink router? If yes then how can I do this? 

If I could just start off by accessing the GUI I might be able to make some headway.

Thanks

[router]

Just bought a USB -C to USB adapter so I plugged in the USB Network adapter with a network cable to my tplink router and now get the following continuously.
ue0: link state changed to UP
ue0: link state changed to DOWN
ue0: link state changed to UP
ue0: link state changed to DOWN


I also tried through a different router with DDWRT firmware configured as a switch with both laptops plugged into the switch but it looks impossible to open the opnsense GUI even though it says I can open the GUI at 192.168.1.1, the other router is set to 192.168.1.2
 Do you have any ideas how to at least access the GUI?
Thanks