PPPoE freezes for 5 seconds every 60 seconds

I have NetGear R7800 router running OpenWRT 21.02, configured with WAN PPPoE interface.

Between the ISP and R7800 router, there is Vodafone Gigabox configured in bridge mode. I need this device because it serves as external modem (VDSL2) for R7800, which does not have integrated DSL modem.

Basically this is my setup:
R7800 (OpenWRT) <--> Vodafone Gigabox (bridge mode) <--> ISP

The issue is that every ~60seconds the traffic is being dropped for about 4-5 seconds. Here is the screenshot from my Android phone:

When using Vodafone Gigabox in a standard mode, the internet connectivity works just fine and such anomaly is not present, therefore I assume this is a bug within OpenWRT.

No logs regarding disconnects/reconnects, PPPoE uptime is fine (today morning it was 12h+) and I tried dozen of different settings and I have no clue what might be causing this. Any suggestions?

I've stopped using this setup and now I am using double NAT. Internet is perfectly stable now. Also using DMZ host in vodafone gigabox router settings which points to OpenWRT router which makes it almost equal to internet facing router.