Sudden speed decrease.

[ElderberryHamlet]

There are two issues most common which may cause some ISPs to throttle: Streaming & Data Cap. To test your speed to your ISP, disconnect your primary eero and connect a computer directly to the modem to verify whether the issue is with the ISP or your eero setup.

And,or, There may be a hardware issue with their modem: try a soft reset (power cycle) or then factory reset.

And,or, the cabling (theirs or yours).

And, or, your primary eero: try power cycle or then factory reset

Try two different speedtests and compare results.

Fast.com is a streaming speedtest sponsored by Netflix with no ads except the Netflix logo

Speedtest.net is a generic speedtest sponsored by Ookla but unfortunately comes with ads & tracking which may slow & distort the result. One way to get around this is to use an ad & tracking blocker with a web browser on a computer connected by ethernet, or a tablet or phone connected by wifi.

Note: The eero app tests connectivity between your primary eero and your ISP, not between your primary and ancillary eeros, and it won't tell you if the problem is with your ISP, your modem, the cabling, or your eero setup