@Sash2025, couple of suggestions if you haven't tried them already.
Disconnect all other local network clients from the router except one wired computer, reboot the router, then test to see if the issue persists with all other local clients disconnected from the router. As suggested by @Ripshod above something else might be interfering on the local network.
If, when, you can access the router GUI check to see if you have enabled the filtering option on who can access the GUI (may be called Enable Access Restrictions on the Administration > System page). Could be the IP address/MAC address on the client you are using to access the router has changed and the router is preventing access. Disable the option and see if the issue persists.
Disconnect all other local network clients from the router except one wired computer, reboot the router, then test to see if the issue persists with all other local clients disconnected from the router. As suggested by @Ripshod above something else might be interfering on the local network.
If, when, you can access the router GUI check to see if you have enabled the filtering option on who can access the GUI (may be called Enable Access Restrictions on the Administration > System page). Could be the IP address/MAC address on the client you are using to access the router has changed and the router is preventing access. Disable the option and see if the issue persists.