Troubleshooting Packet Loss in my Home Network

Question

I have a DSL subscription in my apartment and I want to connect a second apartment of mine in the same building. Here's the setup:

I have a wireless ADSL router that was provided by the ISP: Technicolor TG582n (192.168.1.254). An ethernet cable goes from the router of this apartment to the router of the second one. Its length is between 20 and 25 meters.

In the second apartment, I have a wireless router. Linksys E900. This one I bought not so long ago. This router was set in bridged mode.

I didn't set up the network personally. My brother who set up the network said that the Internet connection from the second apartment is "working really good", before I get to live there. He even played some kind of MMORPG with a couple of friends with no issues.

However, I only encountered issues in the second apartment. I noticed that the connection takes ages to be established, while it's normal in the first apartment. Everything was slow. Browsing was very very difficult.

I ran a a ping from my laptop in the second apartment to the Technicolor router, and I noticed that I had a lot of lost packets (between 35% and 65%). Packets that are not dropped had very very low latency (between 2 and 3 ms). Packets get lost even more if I ping with greater payload (1k for example).

I unplugged the ethernet cable from the Linksys and plugged it in my laptop. No packet loss, no latency, everything is smooth. I am not a networking guy, but I thought well maybe because it's in bridged mode, so I changed the settings of Linksys to act as a normal router: IP: 192.168.2.1, subnet 255.255.255.0, and its default gateway is the Technicolor (192.168.1.254).

I connected my laptop to the Linksys through Wi-Fi again, and the problem is still there. Now I thought it may be a wireless issue, so I connected my laptop to the Linksys with an ethernet cable. No success.

I used another router (an old D-Link; maybe the router itself is not good?), and no success too.

I took the Linksys to the first apartment and plugged it to the Technicolor with a short cable. I plugged my laptop to the Linksys and the problem went away. There is no packet loss.

This is very puzzling: 1- My tests (especially the last one) show that there is an issue with the cable going from the first apartment to the second one: it could be too long, defected, or there is an interference somewhere. 2- When I plug this very cable to my laptop directly, I have no issues.

Is there anything I missed? How can further pinpoint the issue? Why is it working normally when directly connected to a PC but not when I use a router in the middle?