SIP-T21P and SIP-T22P Registration problem in Tunnels

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Hello . Dear Support ,
I have 40 SIP-T21P and 23 SIP-T22P and 3 W52P in our offices but in 2 different locations that connected these office with PPTP tunnel with mikrotik routers.
also i have backup for my tunnels that if one tunnel will be disconnect backup tunnel will be up and when backup tunnel will be up all my yealink phones will be unregistered except W52P and also i have some grandstream phones . and when reboot the SIP-T21P and SIP-T22P they will not be register or if i reboot routers they will not register and only working solution is , i must change network of phones , that mean if they are in DHCP i must give them Static IP Address and if they are in Static IP Address mode i must change to DHCP and they will be work and phones will be registered again.
this is very strange problem please help me.
Note: i do not have a problem with Grandstream IP phone.
Note: i use zoiper softphone and also there is not any problem with it on changing the Tunnels.

I have another question that is there any option to connect with SSH or telnet to SIP-T21P and SIP-T22P ?