It seems that T mobile has many happy custom on the forum. Ease of use and keeping a US number offers added convenience over getting a local SIM in Europe. if you have a phone that is eSIM capable then you can try out the domestic service for free. Gives you a chance to see if the service works well for your needs and in your area. Please do read through the terms and conditions.
I suspect this offer is in response to T-Mobile's unwelcome publicity of a recent and massive security breech, affecting most (all?) customers' accounts. Don't know if they could have addressed the cause(s) so quickly, but this offer is tempting.
The "Network Pass" which is the 3 month free trial doesn't allow for international service as I'm trying now in the U.S. They charge from $5 and up for international coverage with the free plan. I haven't asked if this is available in Europe. The Magenta Max 55+ plan is $45 per line for a two line minimum with no contract, so you can cancel upon return and has free international phone, data and text up to 40GB of hotspot data. Our other plan charges $10/day for international coverage with major caps on data. Our trip is for 20 days and with two devices it doesn't make any sense to do otherwise.