If your 2504 connected to a switch port configured as trunk port then it is easy for you to use another vlan for this 4G dongle connection.
Create a SSID and assign it to vlan X. Makesure 4G modem connected to switchport configured as vlan X as access port. As long as DHCP for vlan X given from your 4G modem clients on that vlan should able to go to internet via 4G modem.
HTH
Rasika
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