Problem with inlet and outlet height for 4Rain flat tank 5000 liters

Hello dear forum community,

I have a problem with the height of the inlet and outlet for my 4Rain flat tank 5000 liters. The outlet to the rainwater shaft is approximately 12 cm lower than the inlet when viewed from the center. The problem is now the tank. Here, the inlet is located in the dome and the overflow is 27 cm apart from the tank’s center. Since I have to accept the height of the inlet, the question now is how do I make the overflow work. I have attached a small drawing for easier understanding.

So here is the system.

The domed shaft is only placed on top of the tank and is held in place by filling it with sand. Since the inflow and outflow of the rainwater are predetermined (laid under the garage and paving stones), I cannot change anything there. I now wonder whether the connection of the overflow would still work (due to the pressure from the height difference) or whether I should connect the inflow to option 1 and the outflow to option 2. But then what about the filtration, and the dome on top of the tank is not sealed since it is only placed on...

In the dome, water comes from the roof on one side, etc. On the other side, the pipe (tube, hose) leads from the pump to the tapping point. The overflow is located on the tank, not on the dome, and this pipe goes to the sewer or an infiltration unit. If the connection, for example to the sewer, is higher than the overflow on the tank, it will not work; in that case, you will need a pump that pumps some water into the higher-lying sewer or infiltration unit at a certain height.

The overflow to the channel is located as shown in the sketch above that of the tank, but below the inlet. However, I can of course also connect the overflow directly, but then the inlet pipe is below that of the tank inlet. The tank is not yet installed.

You write about overflow, inlet pipe, and inlet. Therefore, I have a bit of difficulty following you.

Perhaps we should agree on the following.
In your sketch, consider the pipes or connections from top to bottom.

Inlet (roof, drainage gutter, etc.)
Sewer connection
Overflow at the tank

As I already wrote. If the sewer connection is higher than the overflow and this pipe cannot be laid deeper (as I understood it), then you have to install a pump that keeps the water level in the tank at a maximum height. I did the same. Only with the difference that I do not pump the water into the sewer but let it infiltrate into the garden.

As an alternative to the pump, if possible, one could also install a soakaway system.

Yes, this is the correct sequence for the connection.

What kind of pump could or should this be? Unfortunately, infiltration is not allowed as an overflow; the discharge must go into the sewer.

I have thought of the following solutions:

1. Idea: Glue the dome firmly onto the tank and use the second inlet as an overflow. The second possibility is a normal access for the supply line, electric cable pump + pressure line pump, but this could also be positioned higher in the dome (make the hole yourself). Here the problem is, for example, filtration.

2. Idea: Connect the tank overflow to the sewer supply line with a kind of siphon (like a washbasin) or directly where the water should actually be pushed out, since the inlet is higher than the outlet.