[Question] No mobile network HDC Galaxy S5

[GadgetKen]

T-Mobile NL looks like it is:
2G GSM: 1800 Mhz
3G WCDMA: 2100 Mhz
4G LTE: 2600 Mhz

In theory your phone should be capable of the first two standards.

Suggestions:

If a dual sim phone try swapping the sim card between the 3G/2G sim slot and the 2G only sim slot to see if it will run in the other slot. Thinking maybe the phone won't connect in 3G mode for some reason but might in 2G(or vice-versa). Should work at least for voice/text(data in 2G would be rather slow in edge or gprs if you get the apn right). Or if only a 3g/2G sim slot, there is likely a soft switch in the settings for 2G only mode.

Maybe the phone thinks it's in roaming mode, and you have roaming turned off so it won't connect. Go into the phone settings and see if you can toggle roaming to "on"

Try a different sim card to see if it's a sim card rather than a phone issue? Maybe try a throw away prepaid sim card for KPN, Vodaphone or Telfort.

Try a T-Mobile store and see if they can get it to work? Maybe they might have to move your account to a new sim card if it won't work on your existing sim card for some reason (like the sim card is locked to your current device IMEI; it's rare these days but I've seen that on some US devices like the defunct Peek email data device on T-Mobile USA).

Contact your merchant and see if they have some suggestions or will accept a return for a defective device?

Good luck!