Frontend not accessable (after 1 year offline operation, IP is available, problem with the router?)

Hi everyone,
I have been running a Raspberry PI with the Home Assistant operating system since March 2021. The Raspberry PI is in a holiday apartment where there is no internet access. A fiber optic connection is currently being laid, the Rasperry has been offline since then and has not been updated.
Everything has been going well so far (Zigbee-Sonoff switches operate the electric heaters, motion detectors register that the apartment is unoccupied, etc. All sensors, switches are operated with a Conbee2 dongle via ZigbeeII), since yesterday the web server (GUI) is no longer accessible .
As a router (without an internet connection) I use a very old Tplink WR841N (V9, current firmware update is from 2013!!!).
The router assigns the desired IP number (I can ping it), but the Raspberry does not appear on the router web server’s client list).
I have already reset the router and set it up again: no success.
I then took the PI home with me and operated it in my own network (with a fritzbox router 7590 and the Internet). Lo and behold, the web server can be reached again.
I hope it’s the outdated/faulty TP-Link router. Is there anybody with other ideas?
Maybe a corrupt homeassistant_v2.db file due to possibly wrong timestamps?
This is a pretty specific problem, but I’m hoping someone can help me.

If you use any encryption, then a wrong time and date will wreck havoc.
Other than that it might just be a question about getting through a lot of timeouts before the system gives up and continues.

Hi WallyR,
thanks for your prompt reply. Since the system was working fine more than one year (at least 2 Blackouts took place), I don’t think it is the encryption. Of course I used WPA2 for WLAN-Access.
How can I disable it? Remember, the router and raspberry 4 is totally offline (no internet, no ddns access!)
Can you please also be a little more specific what you meant with: “to reduce the time of timeouts before the system continues”.
Thanks.
Chris

It’s not WiFi encryption is was thinking of, but SSL connections.
You are probably fine with wpa2 on WiFi.

Regarding the timeouts then it will be hard to change it everywhere, if at all possible.
Most timeouts are around 3 minutes, but there can then be retries coded in and how many of those there are is hard to know, but eventually it will give up and continue.

My solution:
I Setup the Router with the actual time and date and voila, homeassistant shows up with its Frontend.
I hope that our Internet access will be available soon.
Beside this I am pretty happy with my automatizations because heating costs could bei decreased dramatically and the control for the guests is very comfortable.