Update

After multiple tests, I can say that the workaround with the bridge mode does not work with IPTV. Apparently it was just a coincidence that a link was established in my first attempts.

This means that, as I wrote, I always have to unplug the Ethernet cable from the Archer BE550 and plug it back into another unused Ethernet port on the Archer BE550. Only then or when I restart the Archer BE550, when the MagentaTV One is switched on, does the MagentaTV One get a link and have access to the network and the Internet.

This increases my suspicion that it is an issue with auto-negotiation / auto-sensing between the Archer BE550 and the MagentaTV one.

update 2

IPTV does seem to work. Or rather, the MagentaTV One works a little better when the bridge mode is activated for IPTV.

Today or better since yesterday evening I had IPTV turned off and also MagentaTV One was off too. When I later switched on MagateaTV One, it did not get a link to the Archer BE550. Not even when I plugged the Ethernet cable into a different unused port on the Archer BE550 as usual.

Then, while the MagentaTV One was still on and connected to the Archer BE550 via an Ethernet cable, I reactivated the bridge mode for IPTV and immediately afterward the MagentaTV One had a working link and internet access.

It must be an auto-negotiation / auto-sensing issue between Archer BE550 and the MagentaTV One, because if the Archer BE550 switch to bridge mode on the IPTV function, the MagentaTV One get a link.

To be 100 % sure, I will test the next days an additional Gigabit switch between Archer BE550 and the MagentaTV One. And I have set Ethernet port 3 and 4 on Archer BE550 to bridge mode on IPTV function to switch the MagentaTV One, without changing the setup from the Archer BE550, between this two Ethenret ports if no link will come up.

  @Kevin_Z  thank you for your response.

Yes, that's right. If I connect the MagentaTV One wireless with Archer BE550, there are no issues. The problem only occurs when connecting the MagentaTV One via Ethernet cable for the first time each day. Once the MagentaTV One has established a link to the Archer BE550, the connection is just as stable and have no issues.

I don't know why, but somehow the Archer BE550 seems to work a little differently with auto-negotiation in bridge mode for IPTV than when IPTV is off or is not set to bridge mode but to Internet.

With activated IPTV and the bridge mode on the Ethernet port to which the MegantaTV One is connected, the auto-negotiation does not work perfectly and not to 100 % but a lot better. Most of the time I still have to replug the Ethernet cable on the Archer BE550 to another unused Ethernet port so that the MagentaTV One gets a link. And like I told, if I connect my old Archer C9 (LAN to LAN) between Archer BE550 and the MagentaTV One, then the MagentaTV One immediately has a link to the Archer C9 and the Archer C9 also has a link to the Archer BE550 and ever things works fine too.

There are no problems with my older Sony TV, which is connected to Ethernet port 2 of the Archer BE550 and which also only has a 100 Megabit Ethernet port.

update 3

Today I installed an unmanaged gigabit switch (model EW-A005-E) between the Archer BE550 and the MagentaTV One and deactivated IPTV on the Archer BE550 and switched all Ethernet ports back to the Internet.

The switch established a Gigabit link to the Archer BE550 and the MagentaTV One established a 100 Megabit link with the switch. In this setup, the MagentaTV One received a link to the network immediately and without delay, obtained an IP address from the Archer BE550 via DHCP and was immediately online.

So exactly the same as if I put the Archer C9 as a switch between the MagentaTV One and the Archer BE550.

This confirms my suspicion that there is an issue with the auto-negotiation between the Archer BE550 and the MagentaTV One. The only question is how to solve it so that I don't need an additional switch between the Archer BE550 and the MagentaTV One.

update 4

Today or better this morning I deactivate IPTV completely and the MagentaTV One immediately gets a link and is online.

I still have the same setup as described in Update 3. That means I have the unmanaged switch mentioned above between the Archer BE550 and the MagentaTV One and everything works fine.

update 5

After I had updated the firmware from 1.0.11 Build 20240807 rel.25691 to 1.0.12 Build 20240902 rel.22993(4555), I tested the MagentaTV One again without the switch in between. I had connected the MagentaTV One on Ethernet Port 4 of the Archer BE550 and she did not get a link again. IPTV was deactivated for the test.

When I switched the MagentaTV One from Ethernet Port 4 to Ethernet Port 3 from Archer BE550, she immediately received a link and an IP address via DHCP. The same issue as with the old firmware.  So I reonnected the switch between the Archer BE550 and the MagentaTV One and switched from Ethernet Port 3 back to Ethernet Port 4 of the Archer BE550 again.

To my amazement, the Switch did not get a link to the Archer BE550 but the switch had a link to the MagentaTV One. I switched the switch from Ethernet Port 4 back to Ethernet Port 3 on the Archer BE550 and the switch immediately had a link to the Archer BE550. A notebook I used for an other test was also unable to get a link on Ethernet port 4 of the Archer BE550. The Ethernet port 4 was like dead. Ethernet port 4 was only usable again after I had restarted the Archer BE550.

This strange behavior, that the first used Ethernet port no longer works and you have to restart the Archer BE550, only appears if the MagnetaTV One was off and without power and is directly connected and try to get a first link to the Archer BE550.

Mind you, with the Archer C9 these issue don't exist.