Freedompop branded phone does not see mobile network after switch to Tello

I have old Freedompop branded Android phone LG LS-720 (CDMA/LTE smartphone with SIM card). I used it until recently as GPS in my car but it can not run Waze anymore (not enough free storage and memory), so, I decided to disconnect this FP line and to switch this phone to Tello. I cancelled my account at Freedompop, logged in into my Tello account, went to the option "switch device" (my old device on Tello was very old Android 2.3 phone), entered MEID and SIM number, reset the network (##72786#), updated PRL/Profile and successfully connected to Tello network (voice/text/internet worked OK). After approximately 30 mins suddently the phone lost network connectivity and that's all: nothing helps (called to Tello CS, reset the phone etc.) I could not even update Profile/PRL because this update requires active connection to Sprint network and the phone does not show signal! After few hours of unsuccessful atemps and troubleshooting I desided to connect this phone back to Freedompop.

Well, I registered new account, entered MEID, got new line from Freedompop, reset the network (##72786#), updated PRL/Profile and I got working LTE connection and working FP phone number (internet and VoiP voice calls worked). But, again, in 30 mins network signal went to "no signal" and this phone again has no service.

What can be the problem and what can I do here?