Far from advertised speeds (SXR80/SXS80)

Note - this is all assuming that you're using Ookla (speedtest.net) to get your test results.

I had similiar results with my SXK80 when I first set it up.  Ookla was reporting ~300mbps up/down on mobile devices connected to the main router when standing just a few feet away.  Internet connection itself is ~1000mbps up/down and ethernet speeds were as expected.

What I found was that Ookla seemed to be the issue.  In other words, the test results were inaccurate.

Start by trying a file transfer from a wired device on your LAN to your mobile device and see what speeds you get.  This should eliminate any issues with the testing servers/internet connection and give you a good baseline for what your wifi connection is capable of.  I found that the same devices that were reporting 300mbps down on Ookla were getting ~800mbps during LAN file transfers.

If things look good there start trying other speed tests and see how consistent the results are.  What I found was that while Ookla was reporting ~300mbps, fast.com and Google's speed tests both reported much higher speeds in the ~600-700mbps range.

Finally, try some of the other servers available to you via Ookla.  Ultimately what I found was that the test servers it kept connecting me to were bottlenecking my testing.  When I tested a few other nearby servers I ended up mirroring the results I was seeing using the other testing platforms.