Are you using a dynamic DNS service of some kind? I assume this service is being updated to reflect your current IP. If so, does an nslookup relfect the very same IP? If it does, all you need, I think, is ping allowed on the untrust side and to ensure that the appropriate interface is assigned to this zone.
In your second message you mention not using a static IP. If you're not using a static IP and not employing a method of updating your IP address against your DNS, it is unlikely you'll be able to ping your router.
What interface ip address on the SRX are you trying to ping and from where?
Is your modem in bridge mode so that the SRX wan interface has a public dhcp address or is it simply connected to the private LAN behind the comcast modem router?
When you say DNS issues do you mean you cannot resolve addresses behind the SRX or something else?
By default the dhcp settings on the SRX use the carrier DNS servers for all clients via the propogate command. Thus your entry is not used by dhcp clients. You will need to remove this setting.
You will also need to configure the SRX as a proxy dns server.
And then use that server for the dhcp clients.
@eugene1973 wrote:Any further answering would be greatly appreciated. I have junos 11.47xxx , and DNS proxy isn't an option.
Use a free public DNS service such as https://www.noip.com where You can manually register Your SRX IP with the hostname of Your choosing (You don't get to choose arbitrary domain though).
And then every public DNS server will resolve that hostname.domain.tld to return the IP to the machine You are pinging from.