Well, what needs to be done ;..
Set the LTE as primary route to the internet, and set the wifi connection as secondary connection. (failover) for most of the traffic.
Of course we will check if the LTE is usable (the ISP could have lost connection to the internet) : see : http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1
But also, we have to verify if the monthly quota is reached or not, and maybe set LTE down till the next monthly reset: see: http://forum.mikrotik.com/t/interface-data-quota/79514/13
(If we do not want to run with strongly reduced speed (fair use policy) or with extra charges on the LTE subscription)
Failover and LTElimiter can be combined in one setup, and with multiple checks and conditions.
The action taken when the LTE limit is reached can vary (interface down, lower priority, …). When the reset date is reached, it will be automatically restarted. (if this is enabled)
My exemple … of failover. (without routing marks, so not using the splitting of specific traffic) (LTE limiter is in the first link above)
http://forum.mikrotik.com/t/question-about-recursive-route/136156/1
A lot of techniques come together here, and recursive routing takes time to understand (if ever understood). Try it out step by step. E.g. start with failover, test LTE Limiter, combine in a later step.