Found the issue. I used the service from the repo like this:

But I had to replace @BINDIR@ to /usr/bin.

[Unit]
Description=Fail2Ban Service
Documentation=man:fail2ban(1)
After=network.target iptables.service firewalld.service ip6tables.service ipset.service nftables.service
PartOf=iptables.service firewalld.service ip6tables.service ipset.service nftables.service

[Service]
Type=simple
Environment="PYTHONNOUSERSITE=1"
ExecStartPre=/bin/mkdir -p /run/fail2ban
ExecStart=/usr/bin/fail2ban-server -xf start
# if should be logged in systemd journal, use following line or set logtarget to sysout in fail2ban.local
# ExecStart=/usr/bin/fail2ban-server -xf --logtarget=sysout start
ExecStop=/usr/bin/fail2ban-client stop
ExecReload=/usr/bin/fail2ban-client reload
PIDFile=/run/fail2ban/fail2ban.pid
Restart=on-failure
RestartPreventExitStatus=0 255

[Install]
WantedBy=multi-user.target

@olivierlambert ok, if you don't know the issue, than the issue should be something quite regular to find. I thought it would be system specific. I'll try to find the issue and post the answer here for others.

I decided to order an OPNsense router. I guess that should do the work. Thanks for all the information!