The interface on my cisco switch wont work if I make it trunk but works when it's access port

Question

I have a switch config that when set like this it works:

Interface g1/3
Description Router
switchport access vlan 5
switchport mode access
!
!
!
Interface g1/10
switchport access vlan 5
switchport mode access
!
!
!
Interface Vlan 5
Ip address 10.51.5.2 255.255.255.0

I'm currently plugged into port g1/10. As an access port it works just fine and I can get to my routers interface. (My routers IP is 10.51.5.11) (My Ip is 10.51.5.62)

When I switch it to:

Interface g1/3
Description Router
switchport mode trunk
switchport trunk allowed vlan 5

It no longer lets me through to my router. The reason I want it set to a trunk is because I'm going to allow more Vlans in the future through the port but right now I only need the 1. My switch is a layer 2 Switch that uses 802.1q.

How do I set it to a trunk?