Ping spikes to a number between 600 and 800 after 90 seconds of playing and stays there

Hi everyone,
I recently got new internet installed in my apartment. There were limited options available so i went with the highest speed and best connection i could get, an 18mbps DSL plan from AT&T. I can watch 1080p60 streams on twitch with no interruptions and all general web browsing is very fast. However, when i play Overwatch, the ping stays normal (42ms) for about a minute and a half, then inexplicably spikes to 671 or higher and stays there. Unlike other ping spikes i have experienced, this one will stay at a specific number between 600 and 800 and stay perfectly on that number indefinitely. I should mention that this issue stays the same on both ethernet and wifi connections. Any help would be greatly appreciated.

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