unselect NAT enable, put the MAC address of next router into the NAT passthrough, select the bypass firewall, then update.
my second router is linsys router WRT54G-TM. you can set up the application port as following
net2phone start 5060, end 5061, protocal: both, address: (your ATA's address like 192.168.24.102)
enable:select
another2pap start 5060, end 5061, protocal: both, address: (your ATA's address like 192.168.24.102)
enable:deselect (because one is enough for me)
phone2pap start:10384 end:10482 protocal:TCP, address:(your ata's address) enable:select
update then you can work on your ATA