Vectra Technicolor cga2121 router / bridge + connecting a second router = troubl

I am asking for help because, unfortunately, no one can advise me.
I am connected to a 100Mbps network from vectra using the white Technicolor cga2121.
I tested the speed in router or bridge mode and it is normal. Speedtest shows me the right one
speed is ok. What I know is unfortunately there is no access or login to the Technicolor cga2121 device to change the settings.
I don't know the root password, so I decided to connect a regular router and change the cga2121 operation mode to bridge.
After connecting, my speed drops to 25-50 Mb / s. The speed drops regardless of whether the computer is on
connected with a LAN cable or using the wifi of the second router. I have the same effect when I present
cga2121 to standard router mode. I don't know why this is happening. I have tested a lot of devices and I'm not going to
buy an expensive router because I'm not sure if the quality will improve with the normal speed in the range of 100 Mb / s.
Where could the error be? Bad cables? The cga2121 router is normally connected to the wan of the second router. Something specific
setting? I really don't know. I haven't experimented with settings like MTU. I can't even introduce a lot.
I don't know where to look for a bug.

MichalP2020 wrote:

I don't know where to look for a bug.

Without providing the name of the second router, we also do not know if it is the fault of this equipment or something else.

The last one was Netgear WNR 2000 v5. But I have the same thing with others, e.g. linksys WRT610N v2. I tested 3-5 from this category. I know that these are old devices and they are old, but it should work. Only works under IPv4. I don't know if it matters. Quality Of Service is off.

MichalP2020 wrote:

I know that these are old devices and they are old, but it should work.

These are grandfathers, weak processors, a ridiculous amount of RAM - it won't squeeze any more.
Either a new router with a 100Mb LAN / WAN port, or look for something with 1Gb ports.

But what exactly is braking speed? 100Mbit / s wan. Some are even 1000Mbit / s.

@ MichalP2020
The ports do not matter here, since the hardware router is too weak to send 100 Mb.

There used to be a TPLink 1043NDv1 router, it had 1Gb LAN / WAN ports - so what, when the max max was possible, only 120Mb could be pushed through it.

I do not know how such speeds could be achieved in 2014. The device from vectra also belongs to the old models and somehow gives advice. What router is suitable for me to guarantee adequate data flow? Raspberry Pi 3B + or 4 is enough?