Heaters mounted onto the wall are best for this purpose as they cannot be dislodged. This simple treatment is continued for a week or two after symptoms have disappeared to prevent relapse. If, of course, the results of the test come back indicating that a different antibiotic is required, you should switch over immediately. Cortisone should never be used as it suppresses the immune system of an already compromised animal. You have tried the drops, and they don't seem to be working.
Some infections are complicated.
More than one organism could be involved, necrotic stomatitis could be complicating the infection, acute or chronic pneumonia could be setting in. There are any number of reasons, and whatever the cause prompt action must be taken. Veterinary advice must be obtained without delay as a course of injectable antibiotic will usually be necessary. It is vital to keep the tortoise at a higher temperature for the duration of treatment, this speeds up metabolism and drug distribution and also boosts the animal's immune system. A temperature of around 28-30 degrees is usually recommended.