To date, no WiFi & Cellular models of iPad (including the just announced 2020 iPad Pro models) can support Next Generation 5G Cellular connectivity. Recent Cellular models of iPad, including the 2020 models, support LTE/4G.
5G Cellular should not be confused with WiFi capabilities. Recent iPads all support dual band 2.4GHz and 5GHz WiFi. The new 2020 models of iPad Pro support 802.1ax WiFi (also known as WiFi-6) - also operating at 5GHz.
As you planning to purchase a WiFi Only iPad (that lacks both Cellular radio and GPS capability), 5G Cellular capabilities of iPad are irrelevant to you. Your iPad will be able to connect to any compatible 2.4GHz or 5GHz WiFi network - including any cellular hotspot (typically provided over a 2.4GHz WiFi link) provided by a mobile phone - be it 4G/LTE or 5G.
I hope this clarification is helpful in answering your question.