I can save you some time and say that won't find anything wrong with the traffic - simply because it is irrelevant.
A router should not crash no matter what the traffic is - otherwise it will be a fundamentally flawed system from the onset. It would be somewhat understandable if there were an insane amount of connections because that's realistically going to crap out anything system if they are enough of them - but this is not what rclone does. For rclone it's not even that - it's just basic high throughput (a fair bit less demanding and mostly a congestion and heat issue for a router - excepting any major firmware bugs).
The router apparently can't handle you using your full capacity for an extended period. Unfortunately that is not that uncommon on cheap routers Those are very often just built (in terms of build-quality) to be capable of the small burst-loads of a typical user, not a sustained max load, and some of them will crap out under loads that should technically be well within their specs. Even among the non-crap ones, there's going to be a some percentage of defective/flawed ones that have sub-par stability. These are after all produced en-masse and not stress-tested properly, so you inevitably get a few "monday products" in the bunch. This is exactly why swapping the rotuer is usually the first thing you try (easy potential fix for an engineer if you know the rotuer model line is not inherently crap and ought to be working).
You unfortunately made a strategic blunder in mentioning rclone here. You should have just played dumb and said the router was randomly crapping out rather than elaborate too much. Probably should have warned you about that heh... but decent chance you can just call again and talk to a different guy and try again. Hope they didn't log the conversation in too much detail
A lot of ISPs really suck at this kind of service aspect towards their customers. It really is unfortunate. My own workplace started out being focused on solving problems and having happy customers - and then turned into a corporate middlemanagement hell over time where the only concern was to get "number of calls" on the board regardless of if you actually solved the problem (as that was the only thing the bosses at the top referenced as goal statistics). Real shame
The simplest fix is the one mentioned earlier where you cap rclone to use 90-95% of max throughput. If you have not already done so, do that. This will eliminate congestion and make the routers job much less intense (much more work saved than the 5-10% lost in speed). If the problem is just poor build quality there's a decent chance this will significantly increase stability - and perhaps even remove the problem entirely.