This post and the three posts following it provide a high level blueprint for something a little more complicated than what you want to accomplish, and the main router in the example is using DSA, and your C7 is still on swconfig. So there are some differences configuring the vlans in your network file and connecting your WiFi SSID's to the vlans. I recently set up a swconfig EA8500 for a friend, so I'll post that configuration in a little bit, but this will get you started with the basic ideas.
It is much easier, at least for me, to do this from the command prompt and edit the network, dhcp, firewall and wireless configuration files in /etc/config directly. It can be done from luci, but there are a lot of luci menus to click through to make it all happen.
If you do not know how to get to the command prompt on your router and edit configuration files, there is no time like the present to learn if you are going to keep using OpenWrt 
Click here for more detail to see an example login to edit the network file.
# ssh root@192.168.1.1
root@192.168.1.1's password:
BusyBox v1.35.0 (2023-01-25 15:45:14 UTC) built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt 22.03.3, r20028-43d71ad93e
-----------------------------------------------------
root@EA8500-1:~# cd /etc/config
root@EA8500-1:/etc/config# vi network
If you do not know how to edit text files with vi, some find the nano editor easier to use. Here is how to install that:
opkg update
opkg install nano