Go step by step and test.
1. General
Restart your Router.
Startup in Safe Mode and test.
Launch Safari, enter the URL manually and test.
Restart normally by clicking the “Restart” button at the bottom.
Start up in Safe Mode. https://support.apple.com/en-us/HT201262
2. Disable all extensions and test.
Section: Manage extensions:
https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
3. 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.
https://support.apple.com/guide/safari/use-the-safari-develop-menu-sfri20948/mac
4. Safari can’t open a website: https://support.apple.com/guide/safari/ibrw1077/12.0/mac/10.14
5. Check for adware.
Run the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.
For instructions: Install Malwarebytes for Mac Uninstall Malwarebytes for Mac
Click the “Scan Now” button. Once done, quit Malwarebytes for Mac.
Restart the computer and relaunch Safari holding the shift key down.
6. EtreCheck
If the steps posted above don’t help:
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.