I'm sure that you did this, but I'll mention it anyway:

Open System Preferences and go to Network. Click Assist Me and then click Diagnostics. Follow the prompts and see what happens. I assume that you have long since contacted your ISP.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365