Hello, I'm looking to deploy some devices to go in datacenters for out-of-band monitoring / management.
Connection to the world will need to be via LTE modem, and preferably the hardware is already in a rackmount 1u form factor.
Any suggestions?
Well there is teltonika rutxr1 with a firmware based on 19.07 sdk but no official support and turris omnia with rackmount and lte modem as addon at discomp but at least this one has official support
You didn't say what kind of out-of-band you want, so I'll assume serial ports.
The OpenGear stuff is great, but expensive and not directly OpenWrt relevant.
Or a completely open hardware alternative: https://freetserv.github.io/ Cheaper and more on topic, but lots of work...
EDIT: Yes, and you'd have to integrate the modem yourself on the freetserv but that should be easy if you can build the rest
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)
Part of the purpose is for serial, yes, but I was planning to handle the serial interfaces with another device attached via LAN or USB. For example, the Airconsole TS 4 port could be attached to this
Interesting, but wow that is way more work than I'm looking for 
Roqos RC150, but it got way more of everything, than you need.
Or the Datto DNA-VZ5 - [SOLVED] Datto DNA-VZ5: an exciting misadventure
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.
looks great, but these don't seem to be purchasable? it looks like Roqos supplies them as part of some managed service offering
I don't see a rackmount kit/option for this Datto unit, and it doesn't seem to be easily purchasable by itself
It's a rebranded Lanner box - https://www.lannerinc.com/products/network-appliances/x86-desktop-network-appliances
Also check at 4G LTE Mall, for fitting devices.
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.