I changed the 2.4 GHz wireless radio back to AUTO 20/40 MHz on both the 5500 and 868L last night. My wireless speeds have stayed consistent (45 down / 1.95 up) and are very close to wired speeds when running a speed test.
I also was able to find the Motorola SURFboard SB6180 on Amazon for $43. Just after I ordered it, I got a reply from a buyer on eBay that he accepted my bid of $39.99 for the Motorola SURFboard SB6180.
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So now I'll have two of them. It's always good to have a "back up" modem, right?
I might return the modem I bought from Amazon.