HA! So you don't *know* which tower the 166 Mbps came from.
That is damn fast, and at such a distance. But it does make me think of searching for Walter in On Golden Pond:
https://www.youtube.com/watch?v=j_h8C_rCbgU You know he's out there somewhere now!
Your uploads are suffering with only 5MHz available, but the downloads sure lover that tower. It must be a very underutilized tower. 6:36 PM isn't the hottest time of the night usually (at least around here), but it also isn't the slowest time.
As for a band coming and going, the towers can do that based on congestion, on purpose. Just didn't want you driving yourself crazy with aiming. It *is* possible aiming is the reason too, but your signal stats are very good, so good chance it is the tower doing it. And there isn't much you can do about that, other than block the other bands. One way to get a better idea if it is the tower doing this is to run the at!gstatus? command, during a speed test, between 3 and 4 AM. Or maybe in farm country, 2 and 3 AM would work better. The idea is to run it when most people are sleeping. If it consistently gives you the extra band then, but doesn't during waking hours, then it is probably the tower taking it away in an effort to share resources among consumers.
Not sure how your tower mast is set up, but if if you mark a line on whatever is clamping the mast, to use as your zero, and then draw a line on the mast each time you get a spot you want to keep (that lines up with the zero), then you can get back to it easy enough.
The speed can jump around based on congestion. The RSRP won't. So you *should* get the best average speed when you have it set to the best RSRP (for a given CID). What is theoretically possible is for your antennas to be hearing two towers and getting some interference from the one you aren't connected to. If so, then aiming to the side of your target tower that is on the other side from the interfering tower, could, in theory, lower your RSRP, but increase your speeds. I'd bet more on congestion, however.
You definitely don't want to go off just one speed test or even 10 in a short period of time. If you find multiple towers with good speeds, you could mark them off on the mast (at lowest RSRP) and then set it at one of them and use this tool to automate running the speed tests every N minutes for a day:
https://testmy.net/auto Of course, if you are *using* the connection (phones, computers, etc. connected to it, especially if Windows is doing Windows Updates), that will skew the results. Best would be just a single computer with firewall set to not allow outbound connections except for the speedtest.net app. Or Linux, if you have that available.
Anyway, you'll get an idea of what times of day are best and how much the speed varies at certain times of day, what your top speed is, your minimum speed and your average. Then you can try one of the other towers too. However, to be 100% sure you know what the tests are testing, you'd want to run the at!gstatus? command periodically to make sure you aren't jumping CIDs. That would invalidate the tests somewhat. On the other hand, if you have two towers (or CIDS on the same tower) that close together and *if* they both get great speeds, then you have redundancy built into the system in case one tower goes dark.
Your primary and secondary antenna signal strengths for a given carrier are off by more than a little bit. Either the antennas aren't aimed in the same exact direction (which would be magnified at these distances and with that narrow beamwidth) or the lower one is hitting some obstacles or reflections off metal roofs are affecting it more than the top one. Moving it up closer to the top one may improve the situation.
**CORRECTION: I looked at the RSRP values wrong the first time. The primary antenna is worse than the secondary. So either they aren't in sync for aiming and the lower antenna is pointed better, or you have the top antenna plugged into the secondary antenna port on the router and the bottom antenna plugged into the primary antenna port on the router. More info in my next post on this.
Having said all that, I'm just talking about if you really want to eek out the last Mbps for fun. You are getting signal strengths and speeds that most people only dream about, especially at those distances.
Congrats on an awesome setup! And thanks for sharing the details and the pictures.