1: Change WiFi frequency
Following the
manual, page 11, connect to the web administration for your hotspot.
Choose "Settings" then "WiFi/Basic", page 18 of manual.
Set the Wi-Fi Band to 2.4ghz
Ensure Privacy Separator and SSID Stealth are both set to "OFF"
2: Change to IPv4 only
Following the
manual, page 22, Mobile Network
Choose "Settings" then "Mobile Network/APN"
To the far right of the "Active" profile (most likely this is fast.t-mobile.com) click the drop down for "PDP Type" and to IPv4 (not IPv4v6)
Save changes, restart hotspot and try again.