uCPE or SD-WAN Devices with LTE Support

We have been using cradlepoint routers, but we want a device that has the functionality of a cradlepoint with the ability to run own own applications naively on Linux. cradlepoint does not allow you to run your own apps on the device itself eventhogh the newer cradlepoint has a quad core processor. Does anyone have recommendations for a UCPE or SD-WAN device that: cost about what a Cradlepoint cost ($500 to $750) have built in LTE (or supports a M.2 LTE card) has at least one Ethernet port has remote management, and can load your own application onto? We could probably build out own with a marker board of some sort and a Sierra Wireless LTE card, but what linux operating system would be best for this?