So here's why the VPN tunnel helps:
"if you are located in Australia and want to play in USA, you might encounter lag and packet loss while playing. This is because the gaming data packets have to travel a long distance and through highly congested pathways between your system and the gaming server. However, if you have any of the ping reduction software running, what they will do is reroute those data packets through the closest server destinations and through traffic-free pathways. This way you won’t face Overwatch lag in Australia."
Info taken from this site: https://www.vpnranks.com/complete-guide-to-fix-overwatch-lag/
So running a VPN tunnel for gaming might be the way to go for you, or just swap to a different ISP if they don't want to resolve your issues. It should be on your ISP-provider to fix this, it's most likely the way they send your traffic that results in problems for you.
I am assuming that if you connect to a more "local" server you do not experience issues? For example if you connect to a server in Australia - does the problems dissapear?
However, there might be more to this, I do not claim to be pro at these kinds of questions but I hope I've been of some help. Perhaps someone else has some ideas on other ways to resolve your issues but as far as I know. A call to your ISP or using a VPN would resolve your issues.