Data Issues.....HELP

According to T-Mobile's website, if you purchased the T-Mobile version of the Galaxy Z Flip 4.  The supported bands are: 

GSM: 850 MHz, 900 MHz, 1800 MHz, 1900 MHz; roaming LTE: 1, 3, 7, 8, 13, 18, 19, 20, 25, 26, 28, 38, 39, 40; LTE: 2, 4, 5, 12, 14, 46, 48, 66, 71; UMTS: banda I (2100), banda II (1900), banda IV (1700/2100), banda V (850), banda VIII (900); 5G: n41, n66, n71, n260, n261

Device link: Samsung Galaxy Z Flip4 | 4 colores en 256 GB y 128 GB | T-Mobile

When looking up the bands deployed over in the Netherlands, the compatibility for the Z Flip 4 is fairly compatible.   

Bands 1, 3, 7, 8, 20, 28, 32, and 38 are what could potentially be deployed.  The T-Mobile Z Flip 4 supports 1, 3, 7, 20, 28, and 38 but not band 32.  You could potentially be in a service area where only band 32 is deployed.  

Fuente: List of LTE networks in Europe - Wikipedia

Usually though, it's a matter of T-Mobile's roaming information for roaming partners being out of date.  This is why it is recommended to add the APN of the carrier you are roaming on to to help resolve any issues.  Don't delete the T-Mobile APN.  Just add one for which ever carrier (more than likely T-Mobile NL) to ensure the device has the correct settings to connect to the network.  Some devices also hae a separate setting for data when roaming.  Check your network settings to make sure that not only roaming is enabled, but data roaming is as well, if that setting is there.  

You can find the T-Mobile NL APN settings here:  T-Mobile Netherlands - Netherlands - APN Settings