how can I get my iphone 6 bbm to work without wifi

Did you try to switch off Wi-Fi in the Settings and see if the app also works using your cellular data?

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BBM requires access to the Internet and contributes toward your wireless data usage when you're not connected to a Wi-Fi network. Some wireless service providers offer ways for you to check your data usage so that you know whether your service plan is appropriate for your usage.

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copied from BBM FAQ's

http://btsc.webapps.blackberry.com/btsc/viewdocument.do;jsessionid=AEBF75712C910 00C57B90D09D330D6C2?externalId=KB35414&sliceId=2&cmd=displayKC&docType=kc&noCoun t=true&ViewedDocsListHelper=com.kanisa.apps.common.BaseViewedDocsListHelperImpl