I am also using OpenWRT on an EdgeRouter X (Cable Modem WAN) and the device was going down on a daily basis as you mentioned. Based on the inputs from this forum I upgraded to the snapshot builds in January. While there was not a single freeze since then, the internet connection stops randomly several times a day. I already had it checked by my ISP and the modem and even the WAN port of the ERX are accessible during these outages. So it seems to be a routing problem.
So I just reinstalled 19.07.7 following this guide:
Ubiquiti EdgeRouter X, Loading OpenWrt and performance numbers
This time I also moved the WAN connection to eth4 (according to the instructions this is a tweak, but it is not explained why). The system has been up for a day now without an issue.