Hi ZygimantasBliu,
yes same for me, I tried to create another DCHP over VLAN from Teltonika just for testing, it all works fine: no vlan - 1 DHCP Server, VLANTAG - another DCHP server
the problem is LTE IP Passthrough it only works over the untagged interface: for example if you do LTE IP Passthrough untagged port will recieve the mobile card ip, if you tag the interface of the end device - the device will receive the ip of the vlan dhcp server, so that setup works (the problem is I need that done differently: sip ip - tagged, dhcp cient - untagged)
tried to assign mobileinterface to a vlan interface direclty as well - does not work neither
for comparison Mikrotik routers alow to Passthrough the sim ip direclty over vlan or untagged interfaces so that means the modem is a client (of the upstream router) and a server (passing the sim ip) at the same time
is that something can be done about rutos in that regard please, pretty sure its a useful configuration, for example your "gateways" only have 1 port :)