You have to check multiple locations in the “Settings” menu:
- Settings → Wireless & Neworks → SIM cards → Preferred SIM for → Cellular Data
- Settings → Wireless & Networks → Data usage → [select SIM] → Cellular data
- (Should only affect Internet speed and not whether you get an Internet connection at all) Settings → Wireless & Networks → More → Cellular networks → [select SIM card] → Preferred network type (If this is greyed out, put the other SIM card to 2G before.)
- Settings → Wireless & Networks → More → Cellular networks → [select SIM card] → Access Point Names (check with operator)
- Check if a data plan is booked and data is enabled at your provider and that you have not exceeded your limit.
- Check if your provider has a disruption or no network at your place. If possible, try the SIM card in another phone.
- Check if you have no VPN in Settings → Wireless & Networks → More → VPN
- Enable data roaming in Settings → Wireless & Networks → More → Cellular networks → [select SIM card]