How to enable DHCP for dialup IPSec VPN?

I feel like I've checked every guide, but not having success.  I'm trying to have DHCP via tunnel interface so I can assign a static IP to a user.  Users are connecting via native iOS client on iPhone/iPad.  When I have phase1 give out the IP, it works fine, I just can't find a way to reserve them.  Any insight would be great!

config system interface
    edit "MobileVPN"
        set vdom "root"
        set ip 192.168.50.1 255.255.255.255
        set allowaccess ping
        set type tunnel
        set remote-ip 192.168.50.1 255.255.255.255
        set snmp-index 25
        set interface "wan1"
    next
end

config system dhcp server
   edit 4
        set dns-service default
        set ntp-service default
        set default-gateway 192.168.50.1
        set netmask 255.255.255.240
        set interface "MobileVPN"
        config ip-range
            edit 1
                set start-ip 192.168.50.2
                set end-ip 192.168.50.15
            next
        end
        set server-type ipsec
    next
end


config vpn ipsec phase1-interface
    edit "MobileVPN"
        set type dynamic
        set interface "wan1"
        set keylife 28800
        set peertype any
        set net-device enable
        set proposal aes256-md5 aes256-sha1
        set dpd on-idle
        set dhgrp 2
        set xauthtype auto
        set authusrgrp "VPN_Users"
        set psksecret ENC <REDACTED>
        set distance 1
        set dpd-retryinterval 60
    next
end
config vpn ipsec phase2-interface
    edit "MobileVPN_P2"
        set phase1name "MobileVPN"
        set proposal aes256-md5 aes256-sha1
        set pfs disable
        set keepalive enable
        set dhcp-ipsec enable
        set keylifeseconds 1800
    next
end