I don't think that is a supported protocol for ASUS routers. You probably would have to go with a Cisco router that natively supports this. The best you can do is use SIP over TLS if your provider and front end supports this setting in the software.
I use free Grandstream Wave softphones on Android with VoIP.ms configured for TLS (TCP) with SRTP forced... with stock Asuswrt firmware configured on a 2xRT-AC86U wireless AiMesh.
I disable SIP Passthrough on the router out of habit... it caused one-way audio years ago but may not be an issue anymore.