1043ND Ver3 or higher support IPv6,

You mean, with the settings you mentioned, you still can't use POP3 or SMTP? if you use SSL/TSL for your POP3 or SMTP, then you may not using 110 and 25 port. try port 465, 587 or 995

Nice, you are professional:p

May be you could have a try to call the technical support hotline.

1. you could only set one parent PC, but the router won't do any restrict or unrestrict to parent PC or non parent PC, it will only do restrict to "children" PC. "Parent PC" is designed more likely...

How many files you are copying at the same time? If you copy too many files at the same time, it could be a very heavy work for a router.