Ater an update a few months ago, I've had Issues with my FP5 and T-Mobile (Germany) as well, but it was definitely the drivers, as it worked before. The chipset suddenly didn't like the frequencies used by T-Mobile for 5G and lost connection after a few seconds. Fortunately, it was fixed by the next update.
T-Mobile US
fast.t-mobile.com
proxy not set
port not set
user not set
password not set
server not set
MMSC http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS proxy not set
MMS port not set
MCC 310
MNC 260
Auth type not set
APN protocol IPv6
APN type default,supl,is,mms,xcap
APN roaming protocol IPv4
Basically the same as the T-Mobile site except they have MMS protocol as WAP 2.0 and I don’t seem to have that setting.