While the Logic App Custom Connector is best for integrating SOAP APIs into your workflows, you can expose SOAP APIs using Azure API Management instead.
If your SOAP API is on-premises, then you could deploy APIM into a virtual network and provide it access to your on-premises network via a VPN Gateway or ExpressRoute. Do note that this requires the Premium Tier of APIM.
That being said, you could still expose your SOAP API with the Logic App Custom Connector as is by using APIM in front of it. The SOAP API would be defined as-is on the APIM end with it calling the Logic App (that uses the custom connector) instead of the SOAP endpoint directly. And the action in the logic app should be a "catch all" that simply forwards the payload as-is to your backend.
This would be a good option if your SOAP API is on-premises and you are using the On-Premises Data Gateway which is simpler to setup compared to a VPN Gateway. Do note that there will higher latencies in this approach just because of the way requests are forwarded to your on-premises network.