Shocking how little the TM sales people know. That person who informed the OP should be forcibly removed from the front line and sent back to boot camp.
To build on what cavedog posted, and to demystify what the TM sales person tried to tell the OP, I *think* he/she was alluding to the fact that their B593s-601 is unlocked and can run any carrier.
I really like the word "Carrier", while American, because it differentiates the distribution channel from an Internet Service Provider (ISP), or 'service provider'. To the masses this just causes confusion.
Carrier = cellular service companies
ISP= copper/fibre services (ADSL/VDSL/FTTH)
I'm a bit dumbfounded why the sales person would take this stance though, since his/her primary aim should be to sell TM data *along with* the device.
Lastly to the OP, the B593 can only accept a SIM card, not an RJ11 connector for ADSL. And mobile data certainly isn't unlimited, either.