Android 6+ Task Killers List
Asus
Phone Settings > Battery > Battery optimization > Apps > Not optimized > All Apps > Find Telegram and choose 'Don't Optimize'.
HTC
Phone Settings > Power > Battery Optimization > Not optimized > All Apps > Find Telegram and choose 'Don't Optimize'.
Huawei
Phone Manager App > Protected Apps > Add Telegram to the list.
LG
Phone Settings > Battery & Power Saving > Battery usage > Ignore optimizations > Telegram > Ingore battery optimization.
MIUI
Phone Settings > Additional Settings > Battery & Performance > Manage apps' battery usage > Add Telegram to the list.
Motorola
Phone Settings > Battery > Battery Optimization > Not optimized > All apps > Telegram > Don't optimize.
One Plus
Phone Settings > Battery > Battery Optimization > All apps list > Telegram > Don't optimize.
Samsung
Phone Settings > Applications > Special Access > Optimize Battery usage > Find Telegram and make sure that it is not selected.
Sony
Phone Settings > Battery > Battery Optimisation > Apps > Find Telegram and disable optimization.
Xiaomi
Services > Security > Permissions > Autostart > Find Telegram and enable autostart.