I wanted to do the same thing and found I was not able to do so on the modem.
That kick started my homelab adventure because I wanted more control over my home network, which I got all setup and is working the way I want it to...but then while I was working on figuring out port forwarding, I found that they moved that ability along with other stuff into the Xfinity app.
Does your ISP happen to be Xfinity/Comcast?
I wanted to do the same thing and found I was not able to do so on the modem.
That kick started my homelab adventure because I wanted more control over my home network, which I got all setup and is working the way I want it to...but then while I was working on figuring out port forwarding, I found that they moved that ability along with other stuff into the Xfinity app.