TRB142 doesn't have a sleep mode functionality. Our TRB255 on the other hand supports LPWAN since it uses a low-powered module.
When it comes to reducing the boot time, this will depend on how long it takes for the provider to register it on their network. Usually, the modem connection time is between 30-60 seconds because it will scan the available operator, frequency band and selects the best operating frequency within that area. An option that could help to establish a session faster is by configuring an APN directly and not selecting the Auto mode as it comes by default.
An additional option that could help as well to speed up the boot time a little bit is by disabling the services that are unnecessary for your use case. You can check these services in the WebUI of the gateway then navigate to Status→Services.