Reset port forwarding to release port 80

> Model: R7000P|Nighthawk AC2300 Smart WiFi Dual Band Gigabit Router

   Is that accurate?  Firmware version?  Connected to what?

> I was able to set port forwarding on my nighthawk. [...]

   Swell.  How, exactly, did you do that?  Why?  Actual port-forwarding
rules?  Copy+paste is your friend.

> [...] However I made the mistake of using port 80 and so I can't
> access the admin UI anymore. [...]

   "using port 80" _how_, exactly?  Simply configuring a port-forwarding
rule for an HTTP server (port 80) should have no effect on access to
the management web site of the router.

   "can't" is not a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

> [...] Is there a way to access it again, hopefully without resetting
> the entire router.

   If you really did find a way to disable normal access to the
management web site of the router, then a settings reset might be the
only practical way to recover.  Saving a configuration file _before_
making drastic changes can save considerable time and effort.

> I hoped my Synology would have been able to, but unfortunately it
> won't let it as they have been added as custom forwarding rules.

   Huh?  What, exactly, is your "my Synology"; what, exactly, did you
expect it to be "able to"; and how, exactly, won't it let who/what do
what, exactly?