you can actually connect a LTE/UMTS USB stick to the MX64 and use the cellular service as internet connection: https://documentation.meraki.com/MX-Z/Cellular/3G_-_4G_Cellular_Failover
the supported USB-stick list is quite limited, but i've had success with other USB-sticks as well (huawei).
anyhow, meraki treads the LTE (cellular) uplink as "last resort" (sort of) and some stuff is simply not possible. ie the uplink monitoring (the rtt and packet loss graph) only works on the ethernet uplink(s), not on the cellular uplink. and in case there is a cellular + an ethernet uplink, load sharing can't be done between the two.
i placed my wish already that the cellular uplink should be treated like an additional ethernet uplink. there is countries and places in which cellular is cheaper / faster / as reliable as wired internet connections.
nevertheless, you can also use another device to turn the cellular link into an ethernet connection. in that case you may run into issues with clientVPN and port forwarding, as there will be an additional NAT layer, right in front of the MX. some LTE/UMTS-USB-sticks work in a specific mode in which you get exactly the same issue (additional NAT layer), even if you attach them directly to the MX.
… in case the APN of your provider is not supported, just open a case. the meraki-team is very fast in adding it.