Problem with transient on power rails
The transient comes from the control loop of your AC DC converter.
Unfortunately the only way to eliminate this is to tighten up your control loop.
But Why?
The transient only lasts for some microseconds.
This would not cause any damage normally.
A decoupling capacitor across the rails would provide the protection.
A varistor can also be used instead.
Also most likely there is a regulator in your cellular module that will provide enough protection.