Traffic Management Developmental reported working in the TMU without being completely certified due to a sick leave call in.

Narrative

I have been training in TMU for approximately X weeks; and am certified on the TMWC (Traffic Management Weather Coordinator) and TMEC positions. I am not certified on the military coordinator; TMNC; or CIC positions. When I reported for my XX30 shift [in the] morning; the OMIC informed me that the other Controller that was supposed to be opening with me had called in sick; and that I would be on my own until the other TMU CPC arrived at XY30.I opened the TMWC position; and set up the DSP with the ZZZ restrictions that had been published the night before. It is also standard practice for us to schedule to ZZZ1/ZZZ2/ZZZ3 to help control sector saturation in the early part of the shift. I confirmed with the OM that even though I am not certified on the TMNC position; I felt it was best to send out GI messages to the Towers; TRACONS; and Center to initiate the call for release for ZZZ1/ZZZ2/ZZZ3. I have a few hours of training at the TMNC position; and performed the duties to the best of my abilities to try to keep things running smoothly.Prior to XY30; the OMIC informed me that the only other TMU CPC for the shift had also called in sick. We brainstormed to come up with ideas for what other Controllers/Supervisors in the facility might have some TMU experience; and he asked me to notify the command center of our situation. I accepted a MIT restriction for ZZZ1 from the log; sent out a GI message; updated the DSP; and then called Command Center. Luckily; there was a controller in the Command Center that used to work at ZZZ ARTCC and he was able to talk me through some things to help. The TMO came down to TMU and I asked him how he would like me to proceed. I was in a bad spot. It was dangerous for me to operate in positions that I am not certified in; but without that; we would be ATC-zero in TMU. He said that he would take responsibility for anything that went wrong; and to try to do the best I could with what I knew how to do. Fortunately; a little after XY30 a second TMU trainee came in. We have the same certifications except he is certified on the Military Coordinator position and was able to handle that coordination. Although neither of us were certified; we were able to work through the Tracks for the international flights; and a ZZZ Reroute that was sent by the command center.Later on; a third TMU trainee came in. This trainee had been in TMU years before and although he was not currently certified; he had previous experience as a CIC in the area. With weather forecasted for the evening shift; the decision was made not to bring in certified controllers from the night shift to help us on the day shift. The night supervisor came in a few hours early; at XC00 am.I have only been in this area for X weeks; but I have worked overtime every single week since being certified on my first two positions. For weeks; I have been held over and we have worked for hours with no relief to ensure things go smoothly. One CIC sat for ten straight hours without a bathroom break; because there was no one available to relieve him. We are dangerously short staffed; and this event proved that we cannot continue to operate without additional staffing. To prevent re-occurrence of this event; I would recommend hiring additional TMC's; as well as developing a better plan to implement in the event we need to go ATC-Zero in TMU.