My fig tree is smaller than yours and is in a pot approx 26”x26”x26” and will soon need a bigger one.
They need John Innes No 3 loambased compost mixed with horticultural grit at a ratio of about 3:1. The top few inches of this need replacing every year that it’s not potted on.
Figs also need watering every day during late spring and summer ... at least a large watering can full each day.
Feed with tomato feed fortnightly from mid May until mid September.
In early September remove all unripe figs that are bigger than your little fingernail.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Fig trees also lose their leaves in Autumn. We have them in the ground here but I imagine that you'd need a pretty large planter to keep it going. They receive neither fertiliser or spray and are flourishing now. (Fig jam and chutney time).
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Did you water it twice a day in the heatwave?
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.