Ungodly slow internet connection

For a while now I've had unbearably slow, incredibly inconsistent wifi, I do network speed tests pretty regularly and my speed can range from multiple hundred mbps to single digit kbps.

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It gets so slow what it takes like 30 seconds to load a single image, youtube will freeze for upwards of a minute between clicks, and all my videos default to like 240p or 360p.

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There is often a lot of people on the network at once, but even at times where there isn't I notice these problems, I am on wifi but I've previously used ethernet and it didn't solve the issues, the modem is brand new as we just changed plans a few weeks ago because we noticed this problem and the new ISP didn't end up fixing it.

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If anyone has any advice please let me know, I've seen my wifi drop so low it will display NaN for the connection speed.