Hello,
Product code RUT240 *A**** supports the North America Region (Verizon, AT&T, T-Mobile)
Supported frequency bands are as follows:
- 4G (LTE-FDD): B2, B4, B5, B12, B13, B14, B66, B71
- 3G: B2, B4, B5
Here's a link for more detailed information: RUT240 Supported Frequency Bands
Since you were using a different operator even though it's operating under T-Mobile, it seems like it's not compatible with your RUT240. Therefore I recommend getting a sim directly from the T-Mobile operator and test it again.
Best regards,
Robert