Hi and welcome to Digi Forum.
You can bring down the PPP interface off/on by CLI (so via local SSH/telnet connection for example) using the following commands (that will also disable the always on mode otherwise it would automatically re-connect again):
ppp 1 aodion 0
ppp 1 autoassert 0
ppp 1 deact_rq
config 0 save
To bring it back up, the commands would be:
ppp 1 aodion 1
ppp 1 autoassert 1
ppp 1 act_rq
config 0 save
(this will also re-enable the always on mode, depends on your needs of course)
The above suggestions should help in turning the Cellular connection off/on manually via CLI, but there is also another option specific for power saving that is to configure a "power control profile".
At startup, the router always uses power control profile 0 unless otherwise configured, so basically you can configure a power profile that disable the cellular modem (and maybe other options basing on your needs) and just enable that profile by CLI.
More details about power profiles here:
https://www.digi.com/resources/documentation/digidocs/90001019/default.htm#tasks/t_set_power_control.htm%3FTocPath%3DConfiguring%2520system%2520settings%2520%7C_____6
http://ftp1.digi.com/support/documentation/AN_067_TransPort_Low_Power_Mode.pdf
I hope this helps, if you need any further help, please review our support options on our website: https://www.digi.com/support.
Regards
Anny
Digi Technical Support Team