2018 15" MBP + Synology rt2600AC = super-slow 802.11ac. 13Mpbs, MCS=0

Thanks for that.

Yeah, I wish I'd reached out here & to Synology sooner to do more troubleshooting. I'd like to know what it was or to at least provide Synology with some feedback. But with the return window closing, I wanted to take advantage. In general I really liked the 2600 and I wish it had worked. Just unusably slow for even web browsing when stuck in that state, which I'm not tolerating on a brand new router and a $3k Macbook Pro. ;-)

And I agree on the cables - I use a Thunderbolt3/USB-C gigabit ethernet adapter for really serious stuff. There is no substitute for wires. :)

I forgot to mention it, but the "MCS Index: 0" reported in that software above appears to be a bug. It always reports that for any 5Ghz connection on my 2018 MBP. MCS Indexes I'm reporting above are from the OSX Wifi Diagnostics.

Here's a screen grab from right now showing what I mean (with the MBP operating perfectly against the RT @ 1300Mpbs).

The MCS Index in the OSX diagnostics report 0 or 1 when the slow connection is happening.

I have tried channels all over the spectrum, manually setting the TC & RT away from eachother, even forcing them to overlap to see if it's worse. From the lower 36/54 groups to the higher 132 and even the DFS's. Changing the channel disconnects Wifi, which does usually temporarily resolve the issue. But it always returns. I've not observed any difference in length of "good" connection.

The software I'm using doesn't generate a spectral analysis, sorry. My environment is a small-town subdivision. I can see other faint 2.4's, as above. 99% of the time nothing shows up on 5Ghz. Above is a rare 1% time where a whiff of that "Bookend-5G" shows up, but it's very faint. Doesn't seem relevant to speed drop-outs, as above, it's rocking 1300 and that one is fainly visible.

I've had WiFi routers for well over a decade in my current home, relying on 5Ghz since I bought the TC, 4+ years ago. I've never had any issues with the TC doing this. I briefly tried other routers (Google WiFi, Orbi) last year when thinking about upgrading and none exhibited the same behavior. Nothing in my household that I'm aware of has changed in terms of interference-generating things. No new microwaves, DECT phones, satellite, etc. And no other devices seem to be acting strangely either (though none are tasked with moving as much data across WiFi as my MBP).

Very interesting, especially the 802.11n 20Mhz No SGI Data Rate column. 13, 26, 39, 52 - I've seen the OSX TX Rate bounce around on those exact numbers when it's stuck in the super-slow mode. Sort of suggests the MBP + Syn connection is somehow "stuck" on 802.11n in just that 20Mhz slot, rather than 802.11ac 80Mhz like indicated.

Yes, running the latest/greatest SRM 1.2-7742 Update 5. In my ~1 month of ownership I've learned quite a bit about Synology Firmware SNAFU's from browsing their boards. It's quite troubling.

Apple does recommend using MU-MIMO, so I've left that on. I believe the TC had that too. But I had the same thought last week, disabled, made no difference.

Agreed on the 40mhz trial and the factory reset. Hadn't tried that yet. I'm up for trying anything just to see what kind of affect it has. Though if the solution is no 80mhz then the Syno will get returned as then the TC remains the better option since it's done 5/80 reliability for years.

Good ideas guys, I appreciate it!

For fairness sake, Synology Support did finally respond to my ticket. No information other than a request to provide login information so their technicians could remotely access the router to troubleshoot. If I wasn't in the return window I'd let that proceed (after securing/removing a few sensitive devices from my home network...). But with all the gripes on the synology community forums, here, and elsewhere about recent borked software updates, I'm thinking best to return it and try something else when the TC dies.