prolem with failover feature

hi,
can somebody help me to solve the problem with my Astaro.
The server has three cards. one card for lan and two for wan. each wan card is faced to one modem.
When each modem is impaired, due to failover properties all users of the card are passed to the next card.
This has created many problems for users. Every time the Public IP is changed and users are faced difficulty in connecting to the mail box and ...
I'm seeking to solve this problem and want to prevent passing users of one card to another.
what should I do? and what configuration whould be made?
thank you in advance
Julis