Hello you network guru's out there. I'm looking for help with a very strange issue which is happening in my home network.
It might ba a CISCO related, but you are network specialists, you can point me the right direction at least.

Intro:

I have a 4G Internet connection from O2 using their TP-link Archer MR200 router with 20/2 Mbps (Down/Up) connection.
This Archer is connected via ETH to my other router ASUS RT-AC55U in AP mode to extend the Wi-Fi signal across the house and garden.
This works just fine and I have good connection for wired and wireless devices across the house and the speeds are ok.

Recently I decided to "upgrade" (don't laugh! :) ) to wired VDSL O2 connection with speeds 50/5.
To connect this to my existing home network I use an old CISCO 886VA route IOS 15.3.

Now the problem I have:
When I set up the connection from O2 on my CISCO router, I get very good speeds (like 57Mbps / 15Mbps) on the CISCO router itself (show controller vdsl 0).
When I connect my laptop to the CISCO ETH ports and do a Windows 10 Network speed test, I receive 50+Mbps download and 5+Mbps upload speeds regularly. Thats OK.
When I connect my TP-Link router to the CISCO router (set it up to get WAN from WanETH port not from 4G Sim card) and connect my laptop to the TP-link ETH ports, I also get full speed.
But when I try to connect my laptop to the TP-LINK Wi-Fi, I only get 0,42Mbps download and 5+Mbps upload!
I have even tried replacing thet TP-LINK MR200 with another, older wi-fi router I have here (also TP-LINK unfortunately) but its exactly the same 0,42 D 5+ UP. On all wireless devices I have at home.
The strange thing is, when I try to go to a web page while wirelessly connected, it shows the texts, but doesn't load the pictures or other multimedia content.
On the other side, when I try to go to Youtube and play a 4k video, it has no issues whatsoever to play this high quality, bandwidth demanding video stream while I'm wirelessly connected to my TP-LINK.

I'm kind of puzzled here. Don't know where to look for the issues.
I can ping all IP's, I have the DNS set OK, what I'm not entirely sure of is the NAT and why UDP media (video stream from YT is OK, and from other pages it is not even loading up).

I can share the network diagram for you to see the whole picture and I can share the configs if you need them to better understand the setup.

Now I'd be very grateful if anyone can advise where to look for issues with the slow Wi-Fi download speeds. I'm suspecting issues at the TP-Link config, but don't know what to look for.

Thanks