Why are they not being heard? 40% is a huge number. Why is their opinion not represented?
Quite possibly because they don't care as much. I.e. when asked the question, they say, "I'd prefer Catalonia stay a part of Spain", but they don't have nearly as strong of feelings as the pro-secession people. As we can see, they overwhelmingly stayed home.
Similarly, foreigners who don't approve of secession are unlikely to feel strongly about Catalan secession. What they really want is to oppose secession at home. Opposing Catalan secession doesn't really get them closer to that. If anything, it might provide another reason for domestic secessionists to condemn them. Meanwhile, foreigners who support secession can talk in high-minded terms about things like self-determination.
Opposing secession is more dependent on individual issues. For example, if the United Kingdom secedes from the European Union (as is in progress), what actually happens? Do they face higher tariffs? Will their citizens who were EU employees lose their pensions? Will EU countries give UK citizens visas? We've talked about some of those issues in regards to Brexit. That discussion is only starting for Catalonia. The Spanish government has concentrated on constitutional issues rather than issues of practicality.
A foreigner, or even a Catalan native, who opposes Catalan secession may not know what these issues are for Catalonia yet. As such, what would they have to say?
Perhaps Catalonia will have a unity movement where secession opponents can concentrate themselves, making it easier for the media to find them and ask them questions. But until now, opponents seem to have concentrated more on avoiding admitting such a possibility.
Currently, it's easy to find proponents. They're celebrating in the streets and holding signs. When (if) opponents organize like that, we're likely to hear more from them.