REF
https://easylinuxtipsproject.blogspot.com/p/tomato.html
You said
"You might need to connect to a VPN (for example, from your home to your office). In that case, you may have to enable Universal Plug and Play (UPnP) in your router"
That is not true.
#1 You only need to forward ports if you want to allow users from the net to connect through your router to your computer.
#2 For outgoing VPN no forwarding is needed.
#3 And if you need to forward ports (which I doubt you will have to for outgoing VPN), you should not use UPnP.
Stated reason(s) UPnP should not be used:
#1 Just do it manually as it is not that hard.
#2 As well as UPnP goes like this..
If UPnP is enabled in the router
If the app (ex uTorrent) or system (ex Xbox) is set to use UPnP
If the OS (ex Windows) is set to use UPnP
As need be if the software firewall is set to allow UPnP
Then UPnP will open ports in the router automatic.
While that does not seem bad, it is not just automatic - The user with-in the app (ex uTorrent)/system(ex Xbox) is not asked for the router's password! Just horrible/insecure.