I'm following my VPN's tutorial on how to set up their OpenVPN on my VPN router. I download their config file, uploaded to the router, and everything conveniently auto-populates.
However, there is a part of the tutorial that lists settings I should input into the custom config window. The config file I downloaded from their site already auto populates this field (but the info is different than what the tutorial states).
Both seem to connect fine, so which should I use for best/most secure results? Keep the info auto-populated from the config file, or replace it with the text from the tutorial?
If your provider directed you to use the config file and it populates the custom config window. Do so.
A tutorial will be used by any other client, as a template. The config file will be your best source of current custom settings.
If your VPN provider's documentation for routers only talks about OpenVPN, but meanwhile its desktop client software actually supports WireGuard (a much faster and simpler protocol), it may be possible to figure out what settings to use to make that VPN provider work on your router via WireGuard.
If they offer a desktop Linux client, for example, you can temporarily use that client to sign in to the provider and connect via WireGuard, and then use the commands ifconfig and sudo wg show in the Terminal to find out what WireGuard configuration that desktop client is using to connect. If you don't run Linux on your computer already, that's fine. Without affecting your current system, you can install Linux in a virtual machine (VirtualBox is free) or use software like Balena Etcher to make a Live USB to temporarily boot from. The virtual machine method is somewhat more convenient, since it doesn't require rebooting, using physical media, or looking up how to access your computer's boot menu to change its startup disk. I suggest Ubuntu Desktop LTS as an easy-to-use Linux distribution, but you can use whichever distro you prefer.
Once you've obtained the WireGuard configuration for your VPN provider, then you can follow the OpenWrt documentation for how to set up a WireGuard client.
I do this with NordVPN, for example (by following this guide). They refer to their WireGuard implementation as NordLynx.
I should also add (for clarity), that the info in the tutorial is specifically relating to the kill switch functionality. So would that not trump the auto populated info from the config file?
I'm not sure the config file's info it auto-populated was even pertaining to that.
Kill switch functionality is also possible using WireGuard. Here's how to do that on OpenWrt: https://www.ivpn.net/setup/router/openwrt-wireguard/#configuring-a-kill-switch-optional
You expect we have clairvoyant abilities?