Спасибо всем! За двое суток нашел решение:
/ip firewall nat add chain=dstnat action=netmap to-addresses=192.168.50.3 protocol=udp \
dst-address=<public_ip> in-interface=LAN dst-port=5060,5061,10000-20000 \
log=no log-prefix=""
/ip firewall nat add chain=srcnat action=src-nat to-addresses=192.168.50.1 protocol=udp \
dst-address=192.168.50.3 out-interface=LAN dst-port=5060,5061,10000-20000 \
log=no log-prefix=""