I've got the second Internet provider and wanted to use this router in Dual WAN mode. To cut the long story short it's not working. Fail-over ... fails, fail-back ... fails. Load balancing, the way I actually want to use, does not work for a great variety of cases. For instance secure sites. A kind of asymmetrical routing or something. Original firmware and Merlin firmware=same issues. Well known, people complain about this since 2014-2015. Asus does not care.
First of all, the router in question is not a good choice for running OpenWrt at all (both WLANs cards are unsupported[0]).
Loadbalancing between different/ independent WAN connections is always difficult, in practice it only works by distributing the load on a per host basis (as you'd break existing sessions otherwise). So assuming you're bundling two 50 MBit/s WANs and have 2 clients connected to your router, you might (under ideal, theoretical circumstances) get 50 MBit/s for each of them - but never 100 MBit/s for a single client. Even commercial routers ranging into the upper 3-figure/ scratching the 4-figure range (EUR/ USD) won't do a particularly good job at this. mwan3 should do the job (failover at least), loadbalancing with the caveats above. Better solutions would involve either cooperation of your ISP (so they bundle the connections ISP side, rebundling them to a single virtual link) or to rely on a server in a data centre to do this job.
In the end you'll simply have to try it (preferably on better supported hardware), OpenWrt gives you the tools to optimize the settings for your environments, how well they're going to work depends on your requirements.
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[0] the 2.4 GHz WLAN card, BCM4360, has some very basic support via b43, but it's limited to <=54 MBit/s and won't be stable or reliable; this isn't going to improve in the future. the 5 GHz card isn't supported at all.