It’s a routing issue that has been affecting many of us since Blizzard made changes after the recent DDoS attacks.
You won’t get any useful advice here sadly.
The only advice you will get that is even remotely official is that you must go through the impossible process of contacting your ISP on behalf of Blizzard and explain to them the complex technical issues and then have them resolve it for you and your country/region.
I hope you have a degree in network engineering. Good luck!
Ok, but in all seriousness, here’s a bunch of recent posts relating to the problem. If you manage to fix it… let me know 
High Latency - Routing Issues
OW2 latency spikes/lagging 2025/03
OCE Server Lag and Routing Issues
Overwatch is the only game giving me 350+ ping constantly
Ping issues in home servers
High Ping in overwatch
High ping out of nowhere