First, do you have a WiFi+Cellular model of iPad - or a WiFi Only model?
A WiFi Only model of iPad cannot directly connect to Cellular networks - but can use a Mobile Broadband Router (often know as MiFi) to connect to Cellular networks. Broadband Routers are readily available from good computer retailers and Cellular operators (e.g., in the UK, try Currys).
For a WiFi+Cellular model of iPad you can add a DataSIM from your chosen Cellular Carrier. If using a UK Carrier, ensure that “roaming” is both included in your tariff and activated before travelling to a third-country. Depending upon your iPad model, you may also be able to activate a Cellular plan directly from the iPad using its built-in eSIM or AppleSIM:
Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support
Short-term eSIM plans are available and are ideal for travelling - but when compared with an airtime contract with a Cellular provider, the short-term plans are relatively expensive.
If recollection serves me correctly, O2 offer Cellular contracts that include EU Roaming at no additional cost - exactly as was the case before the UK left the European Union. Other Operators (such as EE and Vodafone) may offer something similar.