I was in Ostrava last year, my memory may not be 100%.
The Czech regional integrated tariff systems vary in what tickets are actually offered at regional level.
Moravskoslezsky, ODIS (by the way the web site is kodis.cz, not odis.cz) I think only offer 7+ day season tickets for the whole region, despite the web page you referenced found stating CD accept 24 hour tickets.
My recollection is that multi-operator multi-modal single tickets are zonal, obviously only valid along routes between the two zones purchased.
24 hour and 3 day tickets are available for the Ostrava XXL area which includes Bohumin.
See ODIS tariff page, in English but linked documents are Czech:
www.kodis.cz
This map shows the zonal system, the bright red boundary surrounding blue zones is Ostrava XXL:
The fares table is in Czech
at the bottom you will see a 7 day season for ODIS costs CZK 858 bought online otherwise 987. The only mention of 24h tickets there is for Ostrava XXL as far as I can see.
I used the DPO (Ostrava city transport operator) app to pay for a 3 day Ostrava XXL ticket. Within Ostrava you can use contactless with a daily cap at the 24 hour fare (so sometimes it might be better to buy the 24h).