@Don87 

Hi, welcome to our community.

May I confirm some information to understand your situation better? At the same time, please refer to the suggestions below and share detailed results if you need further assitance.

1. Did you 4G/5G router mode or wireless router mode on the Deco?

2. Who is the ISP? 

3. If a PC connects to the Deco directly via Ethernet cable and the speed is only up to 25Mbps, please test via www.spest.net and share the screenshot result here.

Also, please share the screenshot of the link speed on your PC following this guide. (Please make sure the spec of the Ethernet cable you are using)

4. If the Deco get a private IP address, that means there is another NAT network in the front of the Deco network.That should be the reason of double-NAT.

5.For the unstable connection, please follow the guide to troubleshoot：Unstable connection on the 4G/5G Deco

  @Don87 

Hi, thanks for your detailed update.

Did you set up QoS or parental control on Deco app? Please remove the settings and check again.

Please change another Ethernet cable between your PC and the Deco, then test the wired internet speed gain on PC.

If the speed is still slow, please share the model and OS of your PC.  You may also test speed on another wired device if there is any.

Thank you and best regards~