Hello mpanasci
To be honest, I’m not an expert in IPSec VPN configurations, but this is a problem that happens from time to time. Regarding your experience, it is unfortunately a known issue that promised callbacks get forgotten every now and then.
Also, Sunrise fiber connections run on IPv6 DS-Light by default. If your Fortigate firewall requires a native IPv4 address, the tunnel traffic will probably fail or drop there. You should contact Sunrise support and ask them specifically to switch your connection from IPv6 to IPv4, or alternatively wait from Monday for @Daniele_Sunrise when he is back. Because as I see above, you are still using the older Sunrise Internet Box Fiber, so he can perform this change and maybe even initiate a replacement of your old modem at the same time.
Because when I look at the problem with the modem password, if it works for a short time after a hardware reset with a paperclip but then gets lost again after a few days, that is already very strange. The modem itself might already be the problem. This model is quite old by now and has probably already reached its End of Life phase, which might explain these bugs.
cheerio cosmo