Notice for personal hearing - through GST portal - is valid intimation as per Section 169(1)(d) of the CGST Act, 2017.
W.r.t. time limit prescribed u/s 107 (13), it is subject to "where it is possible to do so".
In any case, I am not sure what the appellant is trying to achieve by arguing on this line against resultant order which already passed by the appellate authority (even if after one year from filing the appeal). No appellant can be interested in successfully arguing that order cannot be passed u/s 107 after one year of filing his appeal, making his appeal itself infructuous.
Section 107 (13) is meant to be used for making an application before appellate authority (for deciding the case) if appeal is not adjudicated despite one year passed from date of filing appeal.
At most & if appellant purses such matter using writ petition route etc., High court / Apex court may impose cost on Dept. / concerned appellate authority (or pass strictures) if it finds that 'delay in adjudication of appeal' is without any reasonable cause. But, such delay will not make order u/s 107 as invalid / time-barred.
Considering above, it is suggested to file an appeal once tribunal gets formed on merits of the case & limitation. And the appellant has failed to avail given opportunity of personal hearing & has also not sought adjournment, it can simply explain reasons behind it while filing the appeal before tribunal.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion.