Thank you for contacting Teltonika Crowd-Support Forum.
I would like to ask you couple of questions to better understand your situation.
Are you able to Ping L2TP server's LAN network through Client's LAN network and vice-versa?
If not, Have you tried adding static route to be accessible via L2TP tunnel network?
Also, Would be please send the topology diagram, what exactly you are trying to achieve. It would be really helpful.
Are you trying to access camera using L2TP server's Public IP?