There could be a couple reasons for the issue.
Firstly are you confident the display is showing the correct temp (is the water actually alot warmer)?
Secondly have you tried covering the hot tub, it maybe the ambient conditions are causing it to lose heat as fast as it producing it, thus not able to get past 26?
Thirdly have you tried filling it with hot water to see if you can get the dial to go past 26 degrees be it use warm water from a kettle or hot water tap?
These troubleshooting steps should hopefully help determine if its a faulty heater, thermostat or something else all together.
Hi, further to my response a few days ago, I just wanted to see if you still require assistance, if you do can you please advise if you have tried manually adding hot water into the tub to see if this affects the temperature or placed a cover of the tub to reduce the potential problem with ambient temperature.