Can I check in for my 90-day report at a different immigration office if I'm away from my home office and online check-in fails?

As a general rule; No 😮 However, your mileage may vary.

Typically you do your 90 day reporting at the office where you got your extension.

SOME offices will let you report with them if you show you're staying in their area when your 90 day report comes due

BUT

not all of them will. 😕

If you're out of town when your 90 days is due and if the online reporting is rejected, you are out nothing by trying to report at the office that serves the area where you stay.

Face it all the can say is no and then when you get back to your home you can go report late and take the 2000baht fine (it doesn't go up whether you're 8 days late or 8 months late 😛 )