The decision came from the Court of Arbitration for Sport on Tuesday after it partially upheld an appeal from the International Tennis Federation (ITF).
"Having already been cleared once, and wholeheartedly standing by the fact that I do not feel that the third offence was committed, I find their decision to try me again and subsequently find me guilty, unfair," Ymer wrote on Twitter. "On top of that, I find it difficult to comprehend that they found an 18-month suspension to be a just punishment."
Ymer, who also stressed that he has "never ... used nor been accused of using banned substances," was initially charged by the ITF in January 2022 after his third whereabouts violation, at a Challenger tournament in France the previous November.
The world No.51 argued he had changed hotel but his agent had not made the necessary update. That was upheld by an independent tribunal, which cleared the player in June 2022.
The ITF appealed, requesting a two-year ban for Ymer. CAS found "the degree of fault of the player, a professional and experienced athlete, was high."
"I understand that these rules have been put in place to protect the integrity of our sport, and that they are there for a reason," Ymer said. "However, I do not believe I broke those rules, and my conscience is clear with God as my witness."
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