@Saint_Kat
Powerline adapters will complement your current setup.
You will need pair of standard no-frills powerline adapters that support Deco Ethernet backhaul. You will place one of adapters in garden pod, and another in the house. The one in the house should ideally be connected with Ethernet cable to second (available) Ethernet port of Main Deco. If you have something already connected to it, tell me what it is and I'll give you other options.
After you've done this, verify the link between powerline adapters is good. You should be able to, using powerline adapters documentation and management tools offered by manufacturer.
As long as powerline adapters can keep good link with each other, that's all you need. In garden pod, connect its M5 with Ethernet cable to powerline adapter. If everything done right, that M5 will report Ethernet backhaul link, example:

Don't expect very fast speeds from powerline adapters, but 100Mbps is feasible and my understanding is it's all you need.
For powerline adapters, I recommend TP-Link brand as they should be compatible with TP-Link Deco mesh, and out of all models they offer choose between (TL-PA7017 KIT) and (TL-PA7017P KIT). Second one, with letter "P" at the end of model name, is for pass-through power socket, which you may find more convenient if you don't have extra electric outlet for both powerline adapter and Deco.
Try powerline adapters. If they can establish good link with each other, this is the easiest solution. If they can't, send them back while they are on return policy and try other options.