Depends. If your ISP has proxy-arp enabled you might be able to connect to things in the /24 subnet, but chances are they have already allocated those addresses, so they'll start seeing duplicate IP messages and they'll know something is up.
If they don't have proxy arp enabled, you run the risk of not being able to access everything in your subnet. You should probably match the config your ISP is giving you.
It will work but chances are your ISP will assigned other /28 from that same /24 to other customers. It will definitely be an issue if you ever need connectivity to any of the other /28 subnets, as your router will consider these addresses as local because of the /24 configured on the interface of your router.
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