A new router: GL.iNet GL-AX1800 Flint
At the end of the afternoon the router I ordered yesterday at Amazon arrived: a GL.iNet GL-AX1800 Flint. Setting up the router was a piece of cake and soon it had replaced the TP-Link TL-WDR4300 with OpenWrt. The latter was causing unexplained disconnects very often. At least, I suspected the router. The Flint is noticeable faster and seems to have a slightly stronger signal.