Fortmatic does support Polygon, but developers need to configure connecting to that network on their app. Until more apps allow you to change networks using Fortmatic, you'll need to follow the instructions below to access funds sent to your Fortmatic wallet over the Polygon network.
1. Export your fortmatic private key (as a backup only)
2. Import the
.json file that gets downloaded into Metamask
3. Connect Metamask to Polygon following this documentation https://docs.polygon.technology/docs/develop/metamask/config-polygon-on-metamask
Note: Similar to how Ether is required to pay for gas fees on the Ethereum network, MATIC is the native token to Polygon. You will need to acquire MATIC tokens in your wallet on Polygon to pay for gas fees to transfer your assets.