This should help:

# save existing ruleset...
ls -lash /etc/sysconfig/iptables*
cp -pv   /etc/sysconfig/iptables{,.$(date "+%F_%T")}
​ls -lash /etc/sysconfig/iptables*

# add NetBackup ports...
iptables -N NBU-IN
iptables -A NBU-IN -p tcp -m multiport -s 10.0.1.0/24,192.168.1.0/24 --dports 1556,13724 -j ACCEPT
iptables -I INPUT -j NBU-IN

# save the tables...
ls -lash /etc/sysconfig/iptables*
service iptables save
ls -lash /etc/sysconfig/iptables*

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In the example above "-m multiport" is used because we are adding a rule for two ports "--dports 1556,13724".

In the example above, two different /24 subnets are being allowed to reach the client.  You may only need one for your configuration.

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N.B:  Some advice.  Read the man page for iptables, as there is a difference between '-A' adding an entry at the tail of iptables, and '-I' inserting an entry at the head of iptables.   i.e. inserting to the top/head means the new rules will be evaluated first, whereas adding (appending) to the the list means they will be evaluated last.   And so, if you were to 'add/append' your new rule after an existing generic drop rule, then your new rule will never be evaluated... and so we avoid this problem by 'inserting' our rule/set for NetBackup Client at the top, so that it is evaluated before any other rules.

One could argue that this is ok *IF* the volume of backup traffic outweighs any other application traffic.  A "purist" admin would check the volumes of traffic for all applications on a server, and ensure that the "order" of rules matches the mostly likely highest frame rates, i.e. the busiest and most chatty and most volumeric applications - remember MB/s is not the indicator for FW rule order, it is ethernet frames/sec over a period of "run window" or period of "application intensity" which should dictate your iptables firewall rule order.