There could be sooooo many things happening, and yes, one could be an infected router.
More common things: CDN that a website uses to push ads has been compromised (we see that from time to time on Macrumors).
Downloaded an ad blocker that gets some of their revenue from the same ad spammers, so, might try a different ad blocker.
The ISP might be injecting ads into the stream from a compromised CDN. To partially remedy that, switch your DNS to Cloudflare or IBM (1.1.1.1 or 9.9.9.9).
Clear your website data to flush anything stuck in the cache.
For the record, using 1Blocker and am not seeing any of the "you've won" spam. Then again, this is about the only discussion board I go on, and all the websites I do visit are not using some of the sketchier CDNs out there and or are well administrated to avoid scam ads.
ADD: here's an old, on-going thread regarding this. And to be fair, if you go through the thread, will see that big time sites like BBC, ESPN, etc. are not immune to their CDNs getting compromised.
https://forums.macrumors.com/threads/congratulations-amazon-user.2100402/