Glad to hear that's working for you. But do be aware that some other old systems might not support prime256v1 either, and you might need to switch to RSA. And at some point (possibly quite a while from now so old Android may not have as much market share), prime256v1 (P-256) keys will be signed by a P-384 intermediate, which Android 7.0 also won't like. RSA is the most backward-compatible, if that's something you're concerned with.
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