There is a lot going on.
First the leaves have prominent dark green veins which usually means some deficiency.
Second the black spots look like septoria leaf spot. The spots are irregular are not necessarily bordered by the veins. Are not concentric and have a halo. Usually as it ages and the spots necrose, the center will drop out. This is a fungal disease.
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There are some yellow patches with stippling, so it is possible that there are sucking pests as well.
Misery loves company, weak plants invite pests and disease.
Weak plants do not produce well. I recommend you not try to save this plant.
In a greenhouse it is important to keep everything sanitary. Which means don't bring sick plants in and sanitize everything on a regular basis. The reason for this is 1) if you don't have double entrance on the door, and you leave the door open, pests can get in. 2) If you close the door, once the pests are in, predators can't get in either. 3)Plants are usually closely spaced in a greenhouse so once you have a disease or pest issue, it spreads like wildfire and usually you will have to take everything out. Sanitize the greenhouse in every nook and cranny and inspect each plant carefully before you bring anything back in. Preferably it is easier to leave the good plants outside and sterilize the pots and start with fresh soil, clean pots, and fresh seeds. If you use transplants, inspect them carefully to make sure you don't bring anything in with them.
It is not practical in my climate to have a glass house. Glass houses are for places that need solar gain. I have plenty of heat, a glass house would be unbearable for people or plants. We do have shade houses or structures with pest netting or a combination of pest netting and shade. Because we don't have glass, and the fabric does breathe, we only need a circulating fan for the larger structure, a small one does not need any fans. We still have to be careful going in and out of the shade structures because pests are waiting for the opportunity to get in, so you have to make sure you close the door behind you fast. I just had that happen. I have bird netting over my garden and I went inside to weed the garden and two birds flew in.
It does help in a hot, humid climate to select cultivars that are appropriate with good disease and heat tolerance. Air circulation is important and water only at the base of the plant. A rain proof roof is nice in the rainy season to help keep the leaves dry, but a fan is better, and good spacing between the plants. Fungal and bacterial spores multiply in moist humid conditions and they are in the air. Be proactive with preventive fungicides in those conditions.
Healthy plants will do better at resisting pests and disease. Your plant has some deficiency. Remember for a tomato plant, get a big container. 16 or 20 inches will accommodate a large tomato. 5 gallons is minimum for a dwarf unless you have Red Robbin or Tumbling Toms which are bred for smaller containers. For a dwarf 2-5 ft tall, the minimum size container for me is 15 gallons. Smaller containers would have to be watered more than once a day and tomatoes have large roots systems and are heavy feeders.
If you are growing tomatoes in containers organically. It can only be attempted in a large container, even then, you will have to supplement weekly. It will probably not really optimize and the yield will be smaller than conventional fertilizer for the same variety in the same soil. I've had issues with nutrient deficiencies growing cucumbers organically even in large pots. Tomatoes were less fussy, but they had issues too. Someone else may have a better plan for growing tomatoes organically in containers. However, you are growing the tomatoes, they need a good healthy start.
In hot humid climates like mine, selecting the right cultivars that are heat and disease resistant is first. Growing a healthy plant, however you choose to do that. To protect against diseases, get cultivars with the disease resistant that plagues you the most. Heat resistant tomatoes are only good up to 95 degrees. Non-heat resistant tomatoes will live with adequate care but will not set in the heat. I just don't grow non-resistant tomatoes June-September. I have a year round growing climate, so I grow the non-heat tolerant tomatoes from September-May. Tomatoes grown during the rainy season need more disease tolerance and more attention to preventive fungicides.
I use tree net bags to stave off pests. It does not stop disease or mites which are smaller than the eye of the netting. And it has to be sealed at the openings and have no holes in the netting. The plants can live in the yard. Shade cloth helps in the heat of summer.
Your tomatoes are probably not producing because you are not growing healthy heat tolerant tomatoes and you are not mitigating the heat if you have a glass greenhouse. Even with a plastic greenhouse, you have to open it up and keep it from overheating. That will let bugs in, so you have to be diligent about that.
Your plants have to be healthy to produce. Work on growing healthy plants and take it step by step. You can get a good harvest of tomatoes. If you are new to tomatoes or gardening, start with easy ones. Cherry and smaller tomatoes have fewer disease issues, produce more, and are more tolerant to heat.
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