weak signal

I have had the 5g home internet for about 6 months now,  started out great,  had excellent signal,  fast speeds,  never any buffering on streaming or anything,  all with 19 devices using it including security cameras,  smart home switches,  outlets, tablets,  TV service but suddenly for the past 2 months my signal in the mornings up until around noon is terrible,  I can't even use my tablet without having to wait forever for images to load,  brings back traumatic memories of dial up, I have ordered an external antenna as t-mobile hasn't seemed to be able to help,  it should be here tomorrow and I will see how that goes,  even in the evenings when my gateway says very good signal the speeds are still a little slow and are sporadic as in up and down and not just a little bit up and down I'm talking from the bottom to the top,  and what is strange is from what t-mobile tells me the network is more jambed from mid day until late evening than it is in the mornings due to so many people using it