Cannot connect through wireless cards but only with phone Nokia N95 modem

[pigtails64]

Hi, I have Sky Broadband which comes with a Sagem router and I have a BELKIN Wireless G Desktop Network Card which for the first 6 months connected fine, although sometimes it took a while to find connection and dropped out a few times but it did connect in the end. It then started failing to find connection at all so started using partner's Nokia N95 (which has a built-in Modem) and has always connected first time and can always see connection. Today I brought a LINKSYS Wireless-N USB Network Adapter with Dual-Band which can see the connection and my computer can now see connection through "MY Network Places" but can still not connect at all but phone is again still working.

A few months ago I also tried another BELKIN card but a much better one to what I've got but again it didn't work and if it managed to connect it was only once and then couldn't find connection again so took it back.

We live in a granny flat built on back of partner's mum's house and router is by her front door as that's where phone line is and the computer is in our flat with one wall in between and is approximately 10-15 metres apart. So I originally thought this was the problem but now because phone has no problem and never has even before when we just used to go on internet on phone not through computer I'm assuming it must be something else.

Can someone please help me as I'm now at a loss as to what else to try and I'd like to be able to use internet whenever I want, not just when partner is at home or without him having to leave his phone at home for me! I have also tried moving router closer in his mum's with longer cables in there but again made no difference, even managed to get it to stretch near our front door but still made no difference!

I don't know whether I need something different from a wireless card as phone says it's a modem?!?!