Public wifi hotspot signal no matter it's AT&T, T-Mobile or Xfinity, etc is always weak. You have to be nearby or have line of sight. Soap box size wifi extender with internal antennas won't cut it.
You will need at least
WISP ( Wireless Internet Service Provider) router which has wireless WAN capability, and put it near a window.
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or maybe something like (not sure)
https://www.amazon.com/TP-Link-CPE210-300Mbps-dual-polarized-directional/dp/B00P4JKQGK
Buy TP-Link 5GHz N300 Long Range Outdoor CPE for PtP and PtMP Transmission | Point to Point Wireless Bridge | 13dBi, 15km+ | Passive PoE Powered w/ Free PoE Injector | Pharos Control (CPE510) White: Electronics - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com
both have AP/ client/ AP router/ AP client router (
WISP) operation modes
Longer distance. However, these are directional.
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Even if above products work (will connect to hotspot properly), don't know if they will solve IPv6/Ipv4 issues.
or
for your smartphone
No experience with any of the above products. Contact the vendors to verify they will work for your situation.
All of the above devices should be placed near a window.