Arcadyan KVD21

I have the Nokia router not the Arcadyan but if the management interface is similar you can get into admin level and then go to each of the WIFI bands. If it has the setting "Enable SSID" then disable this and you should not have the respective WIFI radio operating. If there is no traffic on the WIFI channels on the Arcadyan and you only join to the Linksys WIFI it is probably not going to impact the traffic. The LED signal strength bars really only provide a general idea of signal strength so they are not that informative. If you use the web interface to determine the signal strength of the primary and secondary cellular signals you will have a much better knowledge of what is taking place. If it behaves similar to the Nokia you might try a rotation of the router 5-10 degrees clockwise or counter clockwise to try to improve the signal strength for the 5G channel. With the Nokia is is possible to rotate the router as such and improve the signal strength and quality several - dBm. If you have the SINR/SNR >=20 dBm that is best. Signal quality is really important to improve performance. If you have better signal quality there should be fewer issues with packet damage that would lead to retransmissions. There are plenty of resources on the web to better understand how the cellular metrics reflect how the signals are. 

It might be helpful to contact T-Mobile support and ask them about the behavior. You don't seem to be disturbed by the bandwidth you are receiving so I can only speculate that it does provide a reasonable level of service. That does not seem "normal" to me but it might just be a bug in the firmware for the router that only impacts the LED readout.