So you are basically wanting the ability to have 3 redundant WAN links?
EDIT: For scenarios where we want LTE as our active WAN2 we just use a cradlepoint modem.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
If this was helpful click the Kudo button below
If my reply solved your issue, please mark it as a solution.