Internet down until interface restart, but worked plugged directly to ONT. traceroute packet loss. Cause?

I got a new ISP a week ago and I've been testing the link pinging a dozen hosts each second since a few days.

Yesterday suddenly I couldn't access anything on the internet, pings didn't arrive, not even to my ISP's gateway (in their DC). OpenWrt router seemed to be working as usual, it responded to pings, UI was responsive, it had WAN IP assigned. I tried plugging out the WAN cable and plugging it directly to my laptop, surprisingly to me internet was working on it albeit ping was 4ms higher than usual. I plugged the cable back to my OpenWrt router, internet still couldn't be accessed. I opened LuCI again and noticed the WAN session was about to expire in 12 minutes. I restarted the WAN interface and immediately I could access internet again (ping 2ms higher than normal).

I'm trying to pinpoint the issue to either OpenWrt, my ISP or the ONT (a modem, but for fiber, it's transparent to the network, dumb, no NAT, no features) my ISP gave me. Any ideas?

Perhaps related to this, when I test the network with mtr or traceroute I get high packet loss to my OpenWrt router's IP, but when I ping it, it never has any. What is the cause and how do I make OpenWrt answer pings deterministically (ie. either always or never answers)?

The router is TP-Link TL-WR1043ND v2 with the newest official OpenWrt version. I've been using it for many years, never noticed any problems, never needed to restart it.