Can IPO bids be placed only through the UPI ID linked to the primary bank account at ArihantPlus?

No, you can use any bank's UPI ID to apply for IPOs, but there are specific requirements based on your account type.  

• For individual accounts: You can use any UPI ID to apply for the IPO, as long as the bank account under the UPI is in your name. 
• For joint accounts: You must be one of the account holders in the bank account linked to ArihantPlus. You cannot use UPI IDs of bank accounts in which you are not an account holder to apply for IPOs.  

Whatever UPI ID you use, you need to ensure it belongs to you, has your name and is registered with an approved bank.