router Thomson is WEP you can't change that

I changed the ip address of 192.168.1.254 to 192.168.2.254

You can't do that you need to stay on same sub-net change ip on 5210 to 192.168.1.10
Remember that this is not a guessing game you need to provide info. to be able to understand your needs.
What are your needs to connect to the router thomson, wire, wireless, Repeater, Universal Repeater (borrow internet from a friendly neighbor), Client, AP etc. etc.

fantomas

Do you mean the 5210G works fine with other router except the thomson?

What mode is 5210G working in? Client? repeater?

1) try reset your thomson router and use a wifi password which is not too complex.

2) 5210G is 802.11bg only, your thomson router should work in 802.11bgn mix mode, which is compatible with 5210G

5210G working in Client mode

I mean that
connected only when using WEP
* WPA is not connected to
* something is wrong with WPA encryption mode
**is a problem reading WPA encryption verified by many technical
this happens only with routers thomson
I hope in the future to improve firmware
an antenna with 5210g qualities may have a better result if will have a stable firmware