I’m still not familiar the aspects that may be the problem, so I’m not sure how much help I can be at this point, but if you could do a rundown on what you’ve checked and tested thus far. I’m particularly interested what happened when you tried sims and services of other networks (to rule out anything Deutche telekom related). And did you try a clean PureOS install (regular byzantium or crimson backports) which has untouched settings (as a “reset”)? As a new option, there is the possibility of trying the alternative modem now, which you can set to 4G only.
An why was it that you haven’t contacted Purism service and/or sent them the device?