Unfortunately there is no 14-day change of mind policy available for in-store sales.
As part of the sales experience & terms and conditions this includes that a coverage check should be carried out prior to committing to the plan. Beyond this point, cancelling the SIM would be subject to early termination charges as you have agreed to the terms.
It would only be the store who could make a decision to cancel this contract without charges, but this would be entirely at the manager's discretion due to the above terms.
Retail stores are unable to transact or discuss accounts over the phone, so you would be required to attend in person for security purposes. Again, any decision would be subject to managerial discretion and there would be no obligation to accept a "return".
Signal & network performance can vary depending on the area. No network guarantees perfect coverage everywhere, so the onus is usually on the customer to ensure they have conducted coverage checks & tested the network (through use of a Pay As You Go SIM for example) before committing to a contract term. There are mitigations for homes & building with poor indoor coverage through the use of Wi-Fi Calling.
If coverage checks indicate that performance should be better, that points to the need to undertake some troubleshooting with EE Customer Services to establish the fault.
