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Network Engineer here with a background in wireless.
If you can’t replace your modem and router: Log into your router and change the wifi channel. The channel you are on is likely over crowded. Change the channel multiple times and figure out which one gives you the best performance with something like speedtest.net. Usually the sweet spot is a channel in between 1 and 7 or 7 and 11, depending on what’s going on in your wireless neighborhood.
If you can, replace the incredibly crappy Comcast gear with something like this and maybe something like this(I happen to own both, just FYI). Use the 5GHz band if you can as it’s less populated and make sure your wireless router is positioned away from other objects with the antennas vertically aligned and with maybe some slight variation between them. You’ll love how much better your wireless performance will be!