Edit: Well, I finally got (somehow) to return to the original firmware (and that's not easy, believe me!). For now, it seems to work correctly whether wifi or ethernet switch. The LED "gear" (v8.2) so it's good flashes (when the error occurred, it was either nothing is lit all the time). The button that activates / deactivates the wifi working again on the back cover with the TPLINK firmware (niet with DD-WRT ...).
Still, I ordered (and received) another 841ND (v9 this time). I expect to see 1 to return again if it malfunctions or 2. if the first works ... in short, I touch myself.
I added 2 stars because the config is not more affluent, especially for networked quiches like me! But it is a good product that makes its taf (until proven guilty again).