I'd assume rj45 out of band at least this is what most enterprise grade networking use. Indeed serial access comes handy too at least for initial setups or failed oob (I had an occourance on a juniper qfx)
You should consider a device having an easy access LTE antenna connectors, as well the datacenters - If the buildings are EMI shielded, and in case there is no mobile operator inside, you may need to place an external antenna(s) if permitted. Some racks will block the signal as well. In some cases placing RF devices may not be permitted at all.
You must specify what low power means to you - depending the power consumption most x86 rack-mountable devices may not fit the purpose.
You possibly don't need any 1Gbps ports, since the service inside is likely not supporting the full speeds offered by LTE-A, even the carrier supporting all the features. In your case the LTE category and the maximum upload speed should be more important.
There are some compact MiFi routers that will do the job and offering a battery backup. You can place it somewhere within the rack with a zip tie... These are easily obtainable via the local service providers.
However, if your idea is to run a high-speed VPN towards something purposely built, you may check if the Dell Edge Gateway 3200 has supported hardware - it is purchasable, industrial rated, Dual-SIM, and you can attach sensors to monitor and control everything inside the rack. It is relatively small - it is attachable to a DIN-rail or a DIN-rail rackmount.
My suggestion if staying to 4G is the connection within the important DCs to be tested with some decent mobile phone first, and then buying the devices. If possible go to 5G. Keep in mind that if there is a mobile service inside is either an RF repeater or mobile cell, the latter likely being connected via the same fibre routes and the same power supply as your services.