FULL CONE NAT USING 3CX ON A RPi4

I am using 3CX on a Raspberry Pi4-CM4 and cannot get the server to work using full cone NAT.<br/>
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I have followed all the documentation to the letter but for some reason I cannot stop the router changing the ports (Port Preservation).<br/>
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I have done everything i can think of. SIP ALG is turned off as required but ports 5060 & 5090 seem to be the only ports that will not map properly as they seem to map to ports in the 20000's<br/>
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Any help would be appreciated.<br/>
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Many thanks<br/>
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Michael