1) You haven't stated your existing connection, nor your secondary options, so advice on that front is limited
2) You haven't indicated whether you want auto-failover or are happy to switch things over manually.
3) You haven't stated the size of the operation. i.e. is it really worth it?
Given the lack of commitment to your opening post all I really had at that stage was: LTE Router because it's inexpensive and easy to setup manually when primary fails (providing primary isn't LTE - see point no. 1).