I have a second-hand watchguard as I can set it up thank you
You need to provide a lot more info.
What firewall model do you have?
Have you registered it with WG?
What do you mean by "accessing my computer", and from where?
As Bruce noted, you didn't give enough information to know what you want to do.
Assuming you just want remote access to the desktop, then the easiest thing to do if you need to access it as though you were sitting at the computer is to use a remote access program such as Splashtop, TeamViewer, AnyDesk, etc. I use Splashtop for my business and for access to my clients' computers.
Splashtop link: https://my.splashtop.com/splashtop_referral/AAHHDrXasSVZTV
Whatever you do, DO NOT allow Remote Desktop directly from the Internet without any additional login required. It will get attacked and most likely hacked. That is a major ransomware injection method.
I'd suggest starting by resetting your firewall if you don't know the password or can't access it. You can find directions for all the current firewalls here:
(Reset a Firebox)https://www.watchguard.com/help/docs/help-center/en-US/Content/en-US/Fireware/backup_upgrade_recovery/recovery_procedures_c.html
Once you do this, you should be able to do a basic set-up for your device by using the quick start guide for it.
(Quick Start Guides)https://www.watchguard.com/wgrd-help/documentation/quick-start-guides
WatchGuard Customer Support