Thank you for your question.
Please provide information with response of sms command getgnss and getgps. GLONASS will work only if 4 of one type satellites will be available at one moment. The same rule is for GPS satellites.
In this case my advise will be to use multiple satellite sources.
And for additional information: There is a choice to use only one system between GPS, GLONASS, Galileo or Beidou and it is possible to choose two or three systems together. One exception is that you cannot combine BeiDou and GLONASS systems together. Examples of non-configurable GNSS source combinations are:
GLONASS + BeiDou;
Galileo + GLONASS + BeiDou;
GPS + GLONASS + BeiDou;
GPS + Galileo + GLONASS + BeiDou.
Please provide more information, why do you want to use the GLONASS satellite system only?
Hope this information will help.
Also you can find more information in Teltonika Wiki page: https://wiki.teltonika.lt/view/FMB125_System_settings