Openwrt - problem with nat and sip?

Question

SIP

Router mr-3020. Installed firmware "Cheerful Snowdrop" based on openwrt
acs-house.ru/files/fw-tl-mr3020-44570-3g4g-modems-p
Not a single SIP device behind the router works with this firmware, even software or hardware. More precisely, SIP - works only via UDP (there is no authorization via tcp and nothing reaches the server), but rtp does not rise to any.
Debug from the server side shows that nothing comes from the sip client, it is sent to it but does not reach. When trying to run an echo test, the server sends an audio greeting to the client and launches the echo test application itself, in the client it's like a 30-second call attempt (rtp timeout) Where to dig?
When you change the firmware to the factory, everything works. When connected through another router, everything works. When connected directly to a 4G modem (which is in the router), everything works.
Obviously it does not work through the firmware, but it is needed because of its functionality ..
Forgot to add. After raising on the openvpn router (with the passage of all traffic), sip did not work directly, but it did work through STUN. Tried STUN before - didn't work..

Answer 1

iptables -t nat -L -vn | grep 5060
iptables -L FORWARD -vn | grep 5060

XZ is it in your "snowdrop"