@WelshGasman OK, now the network topology is clear. And yes, the HW v11 doesn't have operation mode implemented - this was introduced in HW v13. First update the FW on all devices and test the...

@Franco3602 Any gigabit router would do the job - for example Archer C64 is a budget gigabit router.

@Vijeth You can't access the RE wireless network or you can't access the RE web management interface ? If the latter try setting a static IP address for your RE so you'll always know its IP address.

@Trisha555 Reset the RE by pressing its reset button for about 10 seconds and use this video guide. If your main router doesn't have a WPS button use this video guide.

@Ozy941 Assuming that RE550 doesn't support IPv6, try disabling IPv6 on you router and check if these apps will work on your phone.

@WelshGasman Can you post a network topology diagram on how these are connected together (what operation mode they are - for HW v14) ? The first thing I would do is to update the FW of all 4 router...