Replace ISP DNS by custom ones

Hello,

sorry if this is not the appropriate category for this subject.

Recently, my ISP had issues with his DNS. So I tried some custom ones (other than the Google ones).

Their performance is limited: DNS resolution take seconds.

So I would like to cache these DNS queries in the FW: It means replace the ISP DNS by these custom ones, and use the ZyWALL as the only DNS server.

First, can you confirm that the ZyWALL caches the DNS queries?

Also, I've seen we can add custom DNS in the ZyWALL, but we cannot remove the ISP ones. My knowledge is limited on this point: Why we cannot remove the ISP ones?

If I add my 2 custom ones with priority 1 and 2 (the ISP ones can't be edited, so they have priority *), can you explain to me how the DNS resolution is made? My custom ones would answer, but probably less fast than my ISP ones (when they are available…). Does the time to answer is taken into account? Or does the ZyWALL proceed the first one, and if it did not get any answer proceed the #2, etc?

Thanks in advance for your help

Kind regards