I think you've misunderstood. To be fair I did forget to mention I was using either extender in Mesh mode (with another R15.)
But the key point is that they're crashing. It is not about signal quality, it's about the E15, and R15 before it, spontaneously rebooting. Anything connected via them loses connectivity for 2-3 minutes, and if I ping the crashing device from my wired network, they're not available for the first minute or two. I haven't been able to watch the E15 when it crashed, but when I was using the R15 I was around to see it crash, with all lights turn off, one red light for a minute or two, and then the rest slowly come back.
(I did answer the other question you asked in the original post - yes, I did a factory reset after upgrading the firmware on both devices.)
This literally about newly set up E15s (and R15s in extender mode), linked in a mesh network with another R15, with the latest firmware, and factory reset, with defaults set, crashing every day or two. Not the network signal being poor, but actually rebooting.