試試看把APN type 加入 dun, T-moble tethering plan 有限量 35GB

Fill the below fields in the new APN page.

Name: T-Mobile US LTE
APN: fast.t-mobile.com
MMSC: https://mms.msg.eng.t-mobile.com/mms/wapenc
MCC: 310
MNC: 260
APN type: default,supl,mms,dun
APN protocol: IPv4
APN roaming protocol: IPv4

There will be few additional fields but just leave them blank and save the settings. Choose this new APN to enjoy unlimited tethering on your T-Mobile network.