On my home server I run several nginx-proxy Docker containers, and I also make them accessible from outside on my own domain.
Of course, on the fiber router facing the WAN I have ports 80 and 443 open, and connectivity from outside is confirmed.
In this kind of setup, a symptom where access from outside was not possible started happening at irregular intervals
Symptoms
When the problem shows up from outside, access becomes impossible with a 403 Fobidden like the one below
XDA Developers forum "403 Forbidden"Inside my home local network the connection works without any problem.
Investigating the cause
When the connection works fine on the home LAN but not from outside, I think suspecting the WAN router is reasonable
Incidentally, the router in my house is a PR-400KI
While working on the problem I restarted the router, and confirmed that access from outside became possible, so it does not look like there is a problem with the port forwarding settings
Restarting the router every time the problem occurs is not a good plan either, and I cannot do it when I am away from home
Digging further into the features inside the router, I found that it can be improved by the UPnP NAT情報消去 feature without restarting the router
The solution
As a result of investigating the cause, it appears the connection failure is due to the UPnP NAT information, so I set the router setting (詳細設定>高度な設定>UPnP設定) to UPnPを使用しない, and since then the problem of not being able to connect from outside with Forbidden has not occurred.
Unless you build a server inside your home and exchange traffic with the outside as in my environment, UPnP is a feature that lets games, Apple TV and IoT home devices connect easily, so I think it is normally better to use it.
For UPnP I referred to the article below