If the VPN peer has a dynamic IP address you will need to slighty modify the configuration, specifically on the following aspects:
1. mode aggressive has to be used:
set security policy [Policy_Name] mode aggressive
2. Let the SRX know that its peer has a dynamic IP address but it will authenticate itself with a hostname (juniper.net as exmaple):
set security ike gateway [GTW_Name] dynamic hostname juniper.net
Note this hostname doesnt need to resolve to any address, it is just a value; however it has to be configured on the remote end as the local-IKE ID of that device.
Ref:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28077&actp=search
https://www.fir3net.com/Firewalls/Juniper/srx-dyn.html