What is the second router (the ISP one?)
Also ipq806x or ipq40xx based? (or something quite different?)
Just thinking that there have been reports on ipq806x routers crashing after seeings in the logs a few messages possibly related to jumbo frames.
See e.g.
ipq806x-gmac-dwmac 37200000.ethernet eth0: len 1675 larger than size (1536)
That is pure guess, but an example of weird things that can happen in some conditions. If the ISP modem send jumbo frames that get mishandled by the router, there might be crashes.