4G Failover Solution for Field Employees - Questions about Cradlepoint

I handle the IT for a very small company, and we have groups of employees that work in the field. They are primarily at our clients' offices and typically access WiFi through a travel router. At some locations, the service is temperamental or unreliable. I would like to add 4G failover to deal with any WiFi interruptions. I am looking into Cradlepoint options, but I'm not sure what to use. I would need the WiFi as WAN feature. It seems like the AER1600, IBR600LE, or IBR650LPE would work, but I can't tell what would be best. I did find out that we have some MBR1400s from an old project, so that may be an option too. They are a discontinued model I think. I'm also open to other options if they're worth considering (Meraki, etc.). I don't have a lot of experience in this area, so I apologize for the questions. I appreciate any input/advice.