Hi, it would be helpful if you let everyone know what brand/model of portable freezer you have. As you have said 'freezer' I am assuming it is a compressor model - I don't know of any 3-way models that would work as a freezer.
I have a Waeco CF40 which is approx 14 years old and it has no problem keeping food frozen although I've only used it as a freezer in up to low 30s ambient temps. I have an insulated cover. Keeping the fridge/freezer in the shade with plenty of air-flow helps.