Pol1987
Do you have any tips what's the best and maybe some options for my router??
Please re-read my post, & perhaps you should do some googling on the topic.
I am not familiar with that router or Qnap, so cannot guide you specifically re those.
Others here may be able to advise further.
It's not options you need to set, but you need to decide on your network layout or topology (ie where/which device is your DNS server) & where to add server names for servers on your local network & probably the IP they are on, so that when you type in their names, your request will resolve.
Where you do that is your choice, ie on the router if it supports that, or in sme server hostnames panel. Take a look at that panel in server manager.
If you decide to use sme server as your DNS server, then you probably need to set that in your router, as I assume the router is acting as gateway for your network, otherwise set your workstations & servers network interface settings to use the sme server as DNS server.
Exactly where & how you set it depends on your network & your choices.
Typically the LAN network interface will automatically use the gateway (ie the router) for DNS also, so you have to override that & set the sme server IP instead to be the DNS server (if that's how you choose to do it). So manually configure the Network settings.
Edit:
PS Why not go back to using your sme server as server & gateway, with the modem in bridged mode. Far simpler to manage.