You can put heat tape or heat pad under the cage. It takes time to heat through the plastic.

However I would make sure the old heaters do not work. Are you sure about that? There is little that can go wrong with how Boaphile installs heaters. The power cord can be damaged - inspect that. Check your controller. Easiest way is plug directly into an outlet to test, and leave for at least 2-3 minutes to allow the heat to get through the bottom. Inspect the power cord first of course.

I have 12-yr-old Flexwatt still going. The stuff does not degrade quickly. Any problem you have must be a connectivity issue.
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Mark