@Whocarestp 

Hey mate, looks like maybe Zen as ISP?

Have you thought about taking the fritzbox out of the equation and use the be25 as the router, so connect the ethernet direct from the ONT? 

Only thing that might prevent it is if your using the Fritzbox for digital voice

  @Whocarestp 

Sorry for the confusion, fat fingers. I edited my post.

Im in the UK, Fritzbox is standard router with an ISP here called Zen

Bear with me, I've done a lot of Deco AP only setups, there is some extra info that would help diagnose your issue. 

Understood if you are using some advanced features of the Fritzbox and have a faster than 2.5Gb Internet connection it makes sense to use it.

If thats the case, how about a couple of suggestions. 

1. Make sure the Deco isn't trying to provide dhcp services erroneously, so in Advanced, make sure to disable Smart DHCP mode.

2. Is it possible to connect the main deco to the microtic switch rather than the fritzbox?

3. Once you have the main deco up and working, do the setup of the satellite Deco by temporarily connecting it to the main deco by ethernet directly. Then once all is working, move it to where it needs to go.

  @Whocarestp 

I think it's the MikroTik switch causing the issue. It's a managed switch and tp-link recomends using an unmannged switch for ethernet backhaul.

The Deco Ethernet backhaul feature is based on the standard IEEE 1905.1 protocol. However, we find that some switches, mainly the D-Link switches, will not forward packets based on IEEE 1905.1 protocol, causing all Deco units in a network loop and becoming quite unstable, you might consider changing another switch or contacting switch’s support directly. TP-Link Switches are mostly compatible with our Deco’s Ethernet backhaul.

I did a search for your switch on google and found the following 

No, the MikroTik CRS304-4XG-IN (and MikroTik devices in general running RouterOS) does not natively support or implement the IEEE 1905.1 protocol.

• 1905.1 Protocol: This protocol is typically used for consumer-grade mesh systems (like TP-Link Deco) to manage Ethernet backhaul and automatically map network topology
• Alternative Support: While the CRS304 is a robust managed switch that can handle complex VLAN setups, it does not forward or participate in the discovery/communication packets required for IEEE 1905.1-based systems to detect Ethernet backhaul
• Recommendation: If you are trying to set up a wired backhaul for a home mesh system that requires 1905.1, you should use unmanaged switches that explicitly support the standard, as "managed" business-class switches often drop these packets.

I would try removing the MikroTik switch and go from the fritzbox to the trendnet switch then from the switch to the 2nd deco to see if it fixes the loop back issue and the speed issue  

  @Whocarestp 

So there are two different issues going on. Deco mesh network loops often happen when using Ethernet backhaul, causing switches to block ports due to detected loops. This frequently occurs when multiple Deco units are connected in a loop (e.g., both Ethernet and Wi-Fi active simultaneously) or when connected through managed switches that block traffic

Most managed switches have an option to turn on Loop Prevention which "might" help stop the loop issue but will do nothing for the packet loss. 

If you just purchased your deco units you could return them and try another brand of router or switch out the MikroTik switch for a tp-link switch. You can find a list of switches that work with tp-link ethernet backhaul here 

I did send a video about this to my contacts on the us forum to see if anything else can be done but even if they can update the deco firmware it would take time.