Setting up routing on OpenWRT

[Speedster]

I've got a TP-Link router that I've flashed with OpenWRT. I want to use this router to switch between two LTE modems, one running Rain uncapped off-peak and the other MTN based upon the time of day. I've worked with OpenWRT (and various Linux variations) but can't figure this one out. A simple cron job should do the trick, but for some reason the routing has got me beat. Anyone able / willing to help me get this sorted quick?

 

[Speedster]

So having played around with this I realise I should be able to get this working with a simple cron specifiying the desired gateway. I have hit some hurdles along the way, perhaps someone could give me a helping hand?

In short, hat I want to end up with is to have my router switch between the uncapped rain and "normal" MTN LTE accounts. I currently have a TP-Link TL-MR3420 v2 router running OpenWRT 15.05.1 and two Huawei B315-936s. I've set up the TP-Link with IP 192.168.0.1 and the 315s with 192.168.0.2 and 192.168.0.3 respectively.

I would think the obvious option would be to use the TP-Link for DHCP and DNS but turning off DHCP on the B315s somehow disable my ability to connect to the internet. My guess is this is something to do with DNS / NAT. Any suggestions to fix this?

EDIT: Working now. Just needed to add some dns servers on the TP-Link