OP, are you on a postpaid or prepaid data plan, Simple Choice or an older plan? Have you successfully used it or roam in other Euro countires? As others noted, if it's an older Sim card, it might not be provisioned for (is the Sim card # begin with 8901 xxxx xxxx etc.) Are you tethering to another device ?

Do a quick check on your T-Mo data card's APN settings, if you can access it - make sure it's on fast.t-mobile.com. MCC - 310 and MNC - 260. APN protocol - if it's on IPv6, change it to IPv4 and APN roaming protocol should be on IPv4.

Power cycle - reboot the device & try again. The rest is, most likely, in the hands of T-Mo support.