Inter vlan help on SG series

I am embarrassed but desperate. Switch is in L3 mode. My PM was ready for this yesterday. I’m given 1 IP from an OTA modem that I want to turn into our own network but the SG won’t let me use standard IP route commands. It keeps telling me that I can’t re-use a gateway. No default route command for example. I set an IP route 0.0.0.0 0.0.0.0 and just grab everything on the network. Help articles keep pushing me into a router on a stick or my solution but with both networks a /24. I’ve given all the vlans an IP. Switch can get out to the internet. Our dhcp network I’ve created cannot. Trouble is I can’t change the modem from handing me vlan 1 with a specific IP(mgmt) and vlan 5(data /32) with a specific IP. I only have 1 switch and 8 ports on the switch. I’d be super grateful for any ideas or direction! https://imgur.com/a/Yj6HBHa