Anyone know the secret to get a sagemcom router?

True enough. There is NO one size fits all especially with cellular wireless. Before the signal ever reaches the gateway there are lots of potential obstacles. There are many other potential components in the gateway that could present a problem. Components out of specification or failing, heat sinks not applied properly to insure proper chip cooling, software bugs, compromised solder connections, power delivery or control being inconsistent, improper manufacturing practices leading to quality control issues etc… There are lots of things to consider. The antenna connectors maybe as they are very small and require extreme care to fix the leads to the board. If not just right it would cause resistance and signal degradation. Factors to also watch, in the cellular metrics, are the RSRQ and SINR values. If the RSRQ signal quality is poor and the SINR i.e. signal noise is also poor that will lead to poor performance as packets will possibly be damaged or lost leading to retransmissions that of course leads to delay. A good clean signal with low noise will improve performance. I would say probably more than having a stronger signal. A strong signal that is trashy will not provide a great deal of performance. 

The T-Mobile home internet mobile application may not be a stellar management tool but it is better than nothing. It is good to be familiar with it so when you need it you know where to go. I hope the Sagemcon works out well. If it is working that means you don't have to fight the mobile application.😎