Help diagnosing a VPN problem

Hello everyone,

i am having a weird problem that i would try to understand where it comes from...  but i'm stuck and don't know what to check anymore.

basically: i have two fgt: 1x fgt 60e (v6.2.5 build1142) connected to a PPPoE connection over a radio link (I think the problem is the ISP but i would like to prove/check it).

1x fgt 100d (v6.0.10 build0365) connected to a VDSL2 router (which keeps other vpns UP without problems over the same WAN).

For a good period of time the vpn connected to the firewall with the radio link "worked fine". The only problem i had was that i had set a scheduled reboot of the firewall at 6am because the vpn stayed UP but no longer passed traffic over the tunnel. I never understood the reason for this problem but with this reboot the vpn worked fine all day.

Now there is a new problem during the day... The vpn tunnel is up, but randomly no longer pass traffic over the tunnel.

I'm exhausted...

I would like to point the finger on the internet connection over the radio link ... but how can i prove this ? :( i tried to do a packet capture ... but honestly i need help reading it.