Maybe is a power issue, i had an issue similar problem when i plugged in a Huawei MiniPCI LTE card and enabled onboard wifi on a RB912, this command helped.
You might want to try to increase the boot delay in System > Routerboard and make sure your running the latest OS, include matching firmware.
_That being said…_we use the RB953 a lot with 2 modems, but with only one R11e-LTE-US (with the other modem being a Sierra MC LTE modem). I haven’t tested in a while but pretty sure we never got 2 R11e-LTE-US one to work on the RB953, but we really only wanted one anyway so I can’t say for sure. I think this worked because the Sierra modem is a PPP interface modem, so there is only one LTE interface needed in our case (the Sierra modem comes up under PPP).