Symmetric /fullcone nat - sip registration

i have a thompson 585v7 router , its symmetric nat ( i think)

asterisk can register against my ddi accounts no problem - audio both way on incoming no problem.

real problem is registering a nokia / x-lite sip client - externally.

sometimes i get “failed for wrong password” - occasionally “no matching peer found”

internally is no problem .

would my problems be due to symmetric nat ? - if so , how comes i register with my did accounts ?

any help appreiciated .