First fix whatever is sending invalid SIP packets.
Secondly add a section in sip.conf giving the details of what you want to register with you, or, if you are trying to register, add appropriate register or callback extension lines.
Thirdly, if it still doesn’t work, provide a trace that actually shows an attempted registration.
Also, check on what nat=yes really does, apart from producing a “deprecated” warning, and see whether you really need it.