Since Internet access works elsewhere on the computer, I would focus specifically on what Office 2019 is trying to connect to rather than changing the general network settings again.
In Word or Excel, open File > Account and check the activation status under Product Information.
If it shows Product Activated, please share the exact message Office displays and which action triggers it. A screenshot with any personal information removed would also be helpful.
With that detail, we should be able to narrow this down further.