I think I can answer this.
Dell is responsible for the hardware, T-Mobile is responsible for your connectivity to T-Mobile and billing.
With respect to your repaired netbook, did they return it with a SIM card?
If they did not return the SIM card I would recommend contacting T-Mobile to have them send you a new SIM card, when you get it write down the 890 ..... number written upon the card....you can install it and contact T-Mobile to do a SIM swap (transfer the data of your old SIM into the new SIM)
that should work