The TG100 uses a Quectel UC15-E modem and out of the box defaults to UMTS/WCDMA 2100MHz (Band1) but can be configured for 900MHz (B8) only via AT commands directly to the modem. 

See to make sure you carrier supports this, better yet get confirmation directly from the carrier.

Changing the band can be done within Asterisk CLI in command mode so to change you will need to SSH to the device and enter command mode (How to Log in SSH of Yeastar S-Series VoIP PBX

asterisk -vvvvvvvvvr

Tell the device to use Band 8 / WCDMA 900MHz

gsm send at 2 AT+QCFG=\"band\",128

You will either get back 'OK' or 'ERROR' if an error double check your command is as above. The change will take effect immediately. Quit the Asterisk CLI and SSH session and reboot to make sure the settings stick.

If you want or need to switch back to WCDMA 2100 then use

gsm send at 2 AT+QCFG=\"band\",16

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