Go step by step and test.
1. Restart the router.
Startup in Safe Mode and test.
Launch Safari to test.
Restart normally by clicking the “Restart” button at the bottom.
Start up in Safe Mode. https://support.apple.com/en-us/HT201262
2. Safari > Preference > Advanced
Checkmark the box beside "Show Develop menu in menu bar".
Develop menu will appear in the Safari menu bar.
Click “Develop” to show the dropdown.
Click "Empty Caches" from the dropdown and test.
Safari Develop menu https://support.apple.com/guide/safari/use-the-safari-develop-menu-sfri20948/mac
3. Disable all extensions and test.
Disable or uninstall extensions.
Section: Manage extensions:
https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
4 Download EtreCheck: https://etrecheck.com/maspro and post the report here.
Click, “Free Download” button and then “Click to download” button,
open Downloads folder, click on it to open, and then select ”Open”.
Click on the bouncing EtreCheck icon in the Dock.
“Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.
Click “Share Report” button in the toolbar, select “Copy report” .
Paste the report when you reply.
Note: This is a diagnostic test.
