Ericj3742-
What models are your other Rokus without the problem?
Your Express is a lower end unit that can only connect to the 2.4 GHz wifi band. I am guessing that your Express is the only one that can only connect to the 2.4 GHz wifi band, and that the TV and other Rokus are all using the less crowded 5 GHz band.
The 2.4 GHz band was the first one introduced for wifi, and it also is used by other connection systems such as Bluetooth. It can also be affected by radiation from microwave ovens. Consequently the 2.4 GHz band can be crowded by home automation controls, neighbors' routers, and other nearby devices that can lead to interference and reduced throughput.
You may be able to improve things by changing the wifi channel your router is using in the 2.4 GHz band. The preferred channels are 1, 6, and 11 as these do not overlap each other in frequency, so I'd start by trying these, although depending on which channels your neighbors are using you may find one of the other channels gives better results. You also may be able to reduce congestion by setting your router to use 20 MHz channel bandwidth rather than 40 to reduce channel overlap.
I use a wifi app on my phone that can show me the congestion on each wifi channel, allowing me to choose a less congested one. I check this periodically as this situation can change when neighbors come and go and/or reconfigure their routers.
