Some routers allow more than one client to use port triggering or port forwarding.
Some routers are limited to a single client.
None of these allow multiple clients to simultaneously use port triggering and forwarding.
There's almost always an option or a method for entering a range of ports. Although it's sometimes not obvious and it's often badly documented. And yes, sadly, it can be an oversight (or bug) in user-interface design running from firmware.
Exactly which model ASUS router do you have?
If you're accessing your router configuration through a browser, you might be able to edit the page/script source and manually enter your values into fields. But before going through all that trouble, you should simply clean all browser cache/cookies and try reloading the router page, you should also just try using different browsers (especially yucky Internet Explorer, since it's always the Windows "default"). Any dependencies in your router interface (like Microsoft.NET, VC++, etc) should be documented somewhere, although such stuff is sometimes overlooked in non-Enterprise products, so you may have to research your router/firmware specifics or you may have to simply keep your WinOS fully updated with all the latest-and-greatest components.
Some WinOS editions can display remote devices in the Device Manager. If your router is listed, you can right-click on it and access a "Settings" or "Advanced" tab which typically lists all user-configurable options, including hardware firewall services such as port triggering/forwarding IP subsets. You will need to access your router's Admin login to save any persistent changes, but many routers/switches/gateways do not require login for per-session changes.
A software firewall can provide exactly the same functions as port triggering/forwarding. You might find "life a little easier" if you disable port triggering/forwarding and configure in software instead. Windows Firewall is, astonishingly, one of the most secure options available. Impact of a decent software firewall on system resources (CPU, RAM, etc) is usually minimal and actual network performance (in terms of pings and bandwidths) is usually comparable since a hardware firewall actually imposes a performance hit anyways (which varies between chipsets, products, and firmcode efficiencies - all routers are not created equal).
I suspect you basically want Steam's network access to be disabled until/unless you explicitly open the Steam service. Not sure why, exactly, and it's basically a good network security practice, but it's not strictly necessary. Steam isn't going to hack you. Steam isn't very likely to be hacked by someone who will use it to hack you. And any Steam-based trojans/malware payloads you've got on your machine are going to wait until Steam is connected anyways, so locking Steam out on a per-demand basis wouldn't secure anything the next time Steam gets launched. I suspect you prefer hardware firewall to software firewall for performance reasons, but the difference is really quite inconsequential (usually immeasurable) unless your system or network connection already struggles to meet minimum requirements or your network is tremendously busy with packet handling (like
P2P torrent sharing in background while you play games).
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