@Eloik If you forgot the administrative password, you have to reset the RE to its factory defaults by pressing the rest button about 10 seconds and re-configure from scratch.

@ulululu Try changing the connected port LAN1 to another WR841n LAN port. If that doesn't work, use another ethernet cable and test again.

@Kerrya Connect a PC/laptop to your AX55 by a cable to one of its LAN ports. Here's how to find your router LAN IP address.

@sohouser This modem router is usually provided by ISP and there's no user guide available. Did you try to set Timeout to "0" and check what happens.

@Leray22222 This is a know behaviour - you can mitigate this by using a configuration backup/restore when switching modes.

@Targenor You can update the HW v8 FW to TL-WR841N(UN)_V8_170210 - from here. But I doubt that will bring IPv6 support to your router. If I'm not mistaken the IPv6 is available in TL-WR841N from HW...