If I understand correctly, you want :
your computer -> your transport (over I2P)
your transport -> global backbone (over TCP/Backbone Interface), right?
For i2pd, verify that the SAM interface is enabled (open a browser, type localhost:7070, it should appear at the bottom)
It should be enabled by default. If it isn't, enable it in your i2pd.conf file.
Server-side :
in the conf :
[[Name of your interface]]
type = I2PInterface
enabled = yes
connectable = yes
network_name = your_secret_network_name
passphrase = your_secret_passphrase
restart reticulum. run rnstatus, a long string that ends with .b32.i2p should be there. Keep it.
You do need to configure it as a transport and add some other interfaces to it to connect to the global backbone.
Client-side :
[[Name of your interface]]
type = I2PInterface
enabled = yes
peers = your_string.b32.i2p
network_name = your_secret_network_name
passphrase = your_secret_passphrase
restart and it should normally work.
i2pd will also need a couple of minutes to "warm-up" on your computer everytime you restart it in order to connect to the I2P network. So if you want to directly connect to your transport node, it might not be able to directly reach it.
Give it 10 to 15 minutes.
Now, in case you don't know, I2P is a p2p network.
Normally, each node participates helps the wider network by sharing bandwidth. This helps to have much more node diversity than, for example on Tor.
This also allows to mix your own traffic with that of others and making your own traffic harder to identify as everyone shares their own traffic.
Do know however that others can now you're on the I2P network, but not what you do on it.
It can work with ports closed if you are behind a NAT. However, you won't contribute bandwidth to the wider network. It will also make you less integrated and you won't be able your own traffic with that of others.
Do redirect your ports on your transport node if you can and help the network.
Also a warning : due to it's nature, I2P is slow and your traffic might suffer hickups or disconnections from time to time. If one of the peers your traffic was traveling through suddently disconnects, well, the connection will reset and might time out.
If I have completly misinterpreted what you wanted or was a bit too handholdy, sorry ^^'